U N I V E R S I T Y OF NAI R O BI S C H O O L O F ENGINEERING FEB 540: ENGINEERING DESIGN PROJECT 2014/2015 ACADEMIC YEAR

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1 U N I V E R S I T Y OF NAI R O BI S C H O O L O F ENGINEERING DEPARTMENT OF ENVIRONMENTAL AND BIOSYSTEMS ENGINEERING FEB 540: ENGINEERING DESIGN PROJECT 2014/2015 ACADEMIC YEAR PROJECT TITLE: DESIGN OF A DECENTRALISED WASTEWATER TREATMENT PLANT FOR NYARI ESTATE CANDIDATE NAME: MBUCHI EMMANUELLA GATHONI CANDIDATE No.: F21/1727/2010 SUPERVISOR S NAME: DR.F.N. GICHUKI A Report Submitted in Partial Fulfillment for the Requirements of the Degree of Bachelor of Science in Environmental and Biosystems Engineering, of the University Of Nairobi MAY 29, 2015

2 DECLARATION I declare that this project, except where specifically acknowledged, is my original work. This report has not been in whole or in part submitted for any degree or examination at any other University. Mbuchi Emmanuella Gathoni Signature. Date.. This project report has been submitted for examination with my approval as University Supervisor Dr. F.N Gichuki Signature. Date.. ii P a g e F 2 1 / / M b u c h i E. G a t h o n i

3 DEDICATION I dedicate this project to The Almighty God who has been my source of strength during this entire journey and to my parents, Peter Mbuchi and Mary Mbuchi, for their continued encouragement and support. iii P a g e F 2 1 / / M b u c h i E. G a t h o n i

4 ACKNOWLEDGEMENT I am grateful to The Almighty God for giving me strength to overcome all the challenges I faced in my five year journey. I would also like to thank my supervisor, Dr. F.N. Gichuki, for guiding through the whole design process. I thank him for challenging me and helping me be more critical of small details which greatly affect any design process. It has been an honour working with you. My gratitude goes to our course coordinators, Mr. Inima and Eng. Daniel A. Mutuli, for their assistance in choosing the most suitable project to undertake. I thank them for the classes they gave us on The Design Process. These classes helped me overcome the various challenges I encountered during this process. My appreciation goes to the entire teaching staff for coming up with a list of potential project titles. The list really helped in opening up our minds to endless possibilities. I would also like to thank all my lecturers for the lessons taught in the five years. The knowledge gained will assist me in my career. Special gratitude goes to the lab team for exposing me to the practical aspects of the engineering profession. To all my friends and colleagues, thank you for journeying with me through it all. Last but not least, I thank my entire family for their continuous support and encouragement during my lowest moments, and for celebrating with me during the triumphant ones. iv P a g e F 2 1 / / M b u c h i E. G a t h o n i

5 EXECUTIVE SUMMARY Kenya s real estate market is rapidly growing especially on the outskirts of the city with estates coming up. The residents of these estates are forced to have individual septic tanks to deal with their wastewater since they are located far from the main sewer line. This brings the need for Decentralised wastewater systems (DEWATS). Since each person is left to develop their own system, it is difficult to know the effect of such systems on the environment since they are not monitored. The overall objective of this project was to design a DEWAT system for Nyari Estate. This objective was achieved by calculating the amount and quality of the wastewater produced. Alternative systems were then looked into in order to decide on the best option. The decision was based on the type of technology (since this directly affects the capital cost and the operation and maintenance) and affordability. The system is divided into three components i.e. primary treatment, secondary treatment and tertiary treatment. A peak flow of 204m 3 /d was calculated for a design period of 20years. A conveyance path for the wastewater was proposed and the slope of pipes calculated for a velocity maintained between 0.6m/s and 2.4m/s. This is the recommended velocity to avoid excessive corrosion of the pipes while at the same time prevent settling of solids in the pipes. In this case a velocity of 0.9m/s was used. Primary treatment is where most of the settleable solids are removed. It is carried out in a two chamber settler. The length of the first chamber is 5.84m and the second chamber has a length of 2.92m. The width of the chamber is 2.92m and the depth is 1m. Secondary treatment takes place anaerobically in an anaerobic baffle reactor (ABR) with a total volume of 357m 3. It has a width of 7.71m, length of 15.42m and depth of 3m. Tertiary treatment takes place in a horizontal flow roughing filter with three differently sized media whose depth is 1.2m. The width is 5.9m and length is 7m. In pollutant removal, a worst case scenario was assumed for both the ABR and the filter. Since most of the organic pollutants are removed in the ABR, a removal rate of BOD under extreme events was found to be 84.4% which is in acceptable levels of lying between 70-95%. Expected SS found in the effluent after passing through the filter in extreme events was found to be 27.32mg/L which is below the disposal standards of 30mg/L. v P a g e F 2 1 / / M b u c h i E. G a t h o n i

6 LIST OF TABLES Table 1.3: Co-ordinates of Nyari Estate.3 Table 2.1: Recommended parameter ranges in ABR design.23 Table 4.1: Typical pollutant concentrations in raw wastewater 30 Table 4.2: NEMA disposal requirements..30 Table 4.3: Typical Wastewater concentrations.43 Table 4.4: NEMA disposal standards 46 vi P a g e F 2 1 / / M b u c h i E. G a t h o n i

7 LIST OF FIGURES Fig. 1.1: Google Map showing Nyari Estate and proposed site of treatment facility....4 Fig. 2.1: Flowchart of the stages in wastewater treatment. 9 Fig. 2.2: Tilley et al 2014 Schematic of the Anaerobic Baffled Reactor.13 Fig. 2.3: ABR Formwork on Ensobosobo Farm in Ngong showing the access covers...13 Fig. 2.4: ABR construction in Meru Prison.14 Fig. 2.5: Simple diagram of horizontal flow roughing filter 15 Fig. 2.6: Simple diagram of vertical flow roughing filters..16 Fig. 2.7: Components of a horizontal flow roughing filter..17 Fig. 2.8: Flow process layout showing the components in the system 18 Fig. 3.1: Generation of concept design 26 Fig. 4.1: Road network in Nyari Estate Fig. 4.2: Proposed layout of wastewater conveyance Fig. 4.3: Cross-section of two chambered settler.72 Fig. 4.4: Plan view of two chambered settler...73 Fig. 4.5: Cross-section of ABR...74 Fig. 4.6: Plan view of ABR..75 Fig 4.7: 3D diagram of Horizontal Flow Roughing Filter..76 vii P a g e F 2 1 / / M b u c h i E. G a t h o n i

8 LIST OF ACRONYMS AND ABREVIATIONS DEWAT: Decentralised Wastewater Treatment BOD: Biochemical Oxygen Demand COD: Chemical Oxygen Demand NEMA: National Environmental Management Authority ABR: Anaerobic Baffle Reactor HRT: Hydraulic Retention Time BORDA: Bremen Overseas Research and Development Association SS: Suspended Solids EPA: Environmental Protection Agency US EPA: United States Environmental Protection Agency HFRF: Horizontal Flow Roughing Filter VFRF: Vertical Flow Roughing Filter UNEP: United Nations Environmental Programme WASA: Water and Sanitation Agency CAWST: Centre for Affordable Water and Sanitation Technology viii P a g e F 2 1 / / M b u c h i E. G a t h o n i

9 Contents CHAPTER 1: INTRODUCTION Background Statement of the problem and problem analysis Site analysis and inventory Location Topography Area selection Overall objective Specific objectives Statement of scope... 5 CHAPTER 2: LITERATURE REVIEW AND THEORETICAL FRAMEWORK LITERATURE REVIEW Factors considered Treatment stages Flow process Layout Pollutants : THEORETICAL FRAMEWORK Quantity of wastewater Conveyance Line Settler Anaerobic Baffle Reactor Horizontal flow roughing filters Removal of Pollutants CHAPTER 3: METHODOLOGY Generation of Concept Design Problem Identification Site selection Data Collection Population COD, BOD and SS ix P a g e F 2 1 / / M b u c h i E. G a t h o n i

10 3.4.3 Topography Assessment of Alternatives Design of components Primary treatment Secondary treatment Tertiary treatment Performance of the system Drawings CHAPTER 4: RESULTS Objective 1; Determining the quantity and quality of waste water Objective 2: Propose a layout of the wastewater conveyance path (along the road network) Objective 3; Designing the components of the wastewater treatment system SETTLER ABR HORIZONTAL FLOW ROUGHING FILTER BOD, COD, SS removal TABLE OF RESULTS (SUMMARY) CHAPTER 5: DISCUSSION CHAPTER 6: CONCLUSION CHAPTER 7: RECOMMENDATIONS CHAPTER 8: REFERENCES CHAPTER 9: APPENDICES x P a g e F 2 1 / / M b u c h i E. G a t h o n i

11 CHAPTER 1: INTRODUCTION 1.1 Background In both the developed and developing world, wastewater treatment is a major challenge. In fact, priority is given to water supply systems and wastewater treatment lags behind and remains a major challenge in many countries. In most industrialized countries, safe water is supplied to the population and wastewater pollution control has progressed substantially. In the low and middle income countries the situation is different with lower coverage of both water supply systems and sanitation services. Still, water supply coverage is usually higher than the sanitation coverage. In developing countries, for example, 85% of the population has access to safe drinking water, whereas only 32% has access to sanitation services, for example the slum areas (UNEP/GPA, 2000). A similar situation can be found in middle income countries where water supply coverage is higher than sewerage systems coverage. Wastewater management is comprised of wastewater collection, treatment, and reuse or disposal of effluent and sludge (Crites and Tchobanoglous, 1998). It is essential for several reasons: (1) Protecting public health and the well-being of the communities; (2) Protecting the water resources and the environment; and (3) In water-scarce regions, for reuse purposes in order to reduce the pressure from the potable resources (Bakir, 2001; Friedler, 2001). A Decentralised waste water system (DEWATS) is an onsite or cluster wastewater system that is used to treat and dispose of relatively small volumes of wastewater, generally from individual or groups of dwellings and businesses that are located relatively close together. Onsite and cluster systems are also commonly used in combination (US EPA). It is also defined as the collection, treatment and at times reuse of wastewater (domestic) near the point of generation and is not part of the larger main system which is normally the municipal treatment plant. 1 P a g e F 2 1 / / M b u c h i E. G a t h o n i

12 An onsite waste water treatment system is a system relying on natural processes and/or mechanical components to collect, treat, and disperse or reclaim wastewater from a single dwelling. A cluster system is a wastewater collection and treatment system under some form of common ownership that collects wastewater from two or more dwellings or buildings and conveys it to a treatment and dispersal system located on a suitable site near the dwellings or building. The focus of this study will be on cluster systems where I will design a wastewater treatment plant for a group of residential houses. The advantages of DEWATs are that; The recycled water can be re-used It can enhance the property value of an area Simple technologies can be used as compared to central waste water treatment plants They are less expensive to construct and operate than centralized systems and in some cases are less expensive than individual sewage systems Operation and maintenance of Decentralised systems Management is the key to keeping Decentralised treatment systems functioning properly. Management encompasses planning, siting, design, implementation, operation, maintenance, and monitoring the system. After design and implementation of the system, the residents are left with the operation, maintenance and monitoring part. This can be agreed upon by the residents after they decide on what method of management suits them best. 1.2 Statement of the problem and problem analysis Kenya s real estate market is quickly growing with more and more estates coming up on the outskirts of Nairobi which need waste water treatment facilities. 2 P a g e F 2 1 / / M b u c h i E. G a t h o n i

13 The residents of these estates are forced to have their own individual septic tanks since establishing an individual connection to a municipal waste water treatment (WWT) facility is expensive in terms of infrastructure costs. This prompts the need to design a common wastewater treatment (WWT) facility for such communities. The problem is foreseeable and existing. 1.3 Site analysis and inventory Location The study area for the proposed project is Nyari Estate. It is a fairly new residential upmarket estate on the outskirts of Nairobi. The average temperature of the area is C. It borders Red Hill Road, Mwisho Road and Nyari West Drive. It is located between the following coordinates. POINT Easting Northing Table 1.3: Co-ordiantes of Nyari Estate Topography The area slopes from the North to the South. The highest point is at an elevation of 1766m and the lowest is 1750m. The topography is uneven with bumps and depressions therefore the wastewater conveyance line should be underground Area selection The actual site selected for the treatment facility covers an area of 3, m 2. In comparison to the total area of the treatment plant ( m 2 ), the site allows for future expansion if need be. It is located at the lowest part of the estate therefore allowing wastewater to flow by gravity. This also eliminates the need for excessive excavation since the wastewater facility would be installed underground. The area is also accessible via road. The areas was calculated using the area calculator on 3 P a g e F 2 1 / / M b u c h i E. G a t h o n i

14 Fig. 1.1: Google Map showing Nyari Estate and selected site for treatment facility. 4 P a g e F 2 1 / / M b u c h i E. G a t h o n i

15 1.4 Overall objective To design a Decentralised wastewater treatment facility (DEWAT) for Nyari Estate Specific objectives 1. Determine the quantity and quality of waste water. 2. Propose the wastewater conveyance path layout to the treatment facility. 3. Design the components of the wastewater treatment system i.e. a two compartment settling tank, a six chamber anaerobic baffle reactor and a horizontal flow roughing filter and determine their performance. 1.5 Statement of scope The quantity of wastewater produced by the residents of the area was established to determine the volume of wastewater received into the treatment facility. This was based on the current and projected population with a design period of 20 years. After determining the quantity of the waste, the qualitative analysis was done from typical wastewater characteristics and compared to the expected effluent standards based on NEMA standards. A suitable path for conveying the wastewater was done by generating elevation profiles (using Google Earth) of the area (specifically the roads). Using this information, a conveyance layout was proposed for flow by gravity. A treatment option was then selected which best suits the needs of the community based on the cost of construction and implementation, availability of raw materials locally, the operation and maintenance needed and effluent quality required. 5 P a g e F 2 1 / / M b u c h i E. G a t h o n i

16 CHAPTER 2: LITERATURE REVIEW AND THEORETICAL FRAMEWORK 2.1 LITERATURE REVIEW There are different methods available used to treat domestic wastewater. The method selected is based on several factors Factors considered Technical Feasibility The method selected should be technically feasible. The technology should be locally available and easy to implement. Economic viability The method chosen must also be economically viable in terms or cost of building materials and technology used as well as operation and maintenance. Social acceptability Before selection of a particular methods, the social acceptability of it must be considered. It must be socially acceptable to the community in question. Population The population served influences the average and peak flows as well as total inflow into the system. Space available and layout of the buildings The space available for the construction of the treatment facility affects the positioning of the system, its size and the distance (buffer) from the residential area. There should also be available space for expansion if need be. Existing methods used The existing methods popular to the region must also be studied because they will give an insight to what is currently available and the performance Treatment stages The stages through which waste water goes through during treatment are; preliminary treatment, primary treatment, secondary treatment and finally tertiary treatment. 6 P a g e F 2 1 / / M b u c h i E. G a t h o n i

17 Preliminary Treatment This stage of treatment is important where large volumes of wastewater from different sources is being treated. It is extremely important in municipal waste treatment facilities as this waste contains objects such as rags, sticks, floating papers and grease which may cause maintenance or operational problems. Preliminary treatment serves to protect the subsequent stages from elements that may prevent their effective working. It may consist of; Screening Screens are considered an effective and economically efficient method of removing solids and floatables. Generally there are two types of bar screens- coarse and fine. Course screens are constructed of parallel vertical bars and are often referred to as bar racks or bar screens. These screens are usually set at 0 to 30 degrees from vertical and are cleaned by an electrically or hydraulically driven rake mechanism that removes the material entrained on the screen on a continuous or periodic basis. There are three types of bar screens used: trash racks; manually cleaned screens; and mechanically cleaned screens. Fine screens typically follow coarse bar screening equipment and provide the next level of physical treatment in removing the smaller solid particles from the waste stream. There are two types i.e. fixed (static) and rotary screens. The screens are usually constructed of stainless steel in a concave configuration, at a slope of approximately 30 degrees. Grit chamber Grit chambers are basins designed to remove inorganic particles and prevent damage to sensitive components such as pumps. They also serve to reduce grit accumulation in sludge settling tanks. Aerated grit chambers incorporate the use of oxygen to lower the BOD of the water. Comminutors Comminutors are devices used in combination with screens or independently, to chop up large floating objects in wastewater into smaller pieces. 7 P a g e F 2 1 / / M b u c h i E. G a t h o n i

18 Comminutors consist of two sets of cutters where one is fixed while the other is moving. The distance between the two sets equal to the size of chopped material required. Preliminary treatment is not necessary in treating domestic wastewater alone because the sink sieves prevent large objects from entering the system Primary Treatment This stage comes after the preliminary treatment and it consists of a primary sedimentation tank. This is where suspended solids settle out at the bottom forming the primary sludge which is normally removed by mechanical means Secondary Treatment The secondary treatment stage is where most of the organic pollutants are removed. This stage utilizes either aerobic or anaerobic bacteria to break down the organic pollutants into less harmful components. Aerobic treatment In aerobic treatment processes, bacteria use oxygen to feed on the organic material (which is a food source) to produce carbon dioxide and water, with the production of quantities of extra bacterial mass (sludge). Most aerobic processes require the mechanical addition of oxygen and that can be expensive. Examples of applications of aerobic treatment are aerobic ponds and activated sludge processes. Anaerobic treatment Anaerobic treatment processes take place in the absence of oxygen. Bacteria break down the organic wastes to produce carbon dioxide and methane. This mixture of gases, called Biogas, can potentially be harnessed as an energy source. Anaerobic process produces much less excess sludge than aerobic processes. Examples of applications of anaerobic treatment are anaerobic ponds and septic tanks. Bio-filtration processes Bio-filtration processes utilize a combination of biological and filtration methods to treat waste water. A bio-filter is used in the process. A bio-filter refers to a durable bed of aggregate or discs made up of suitable inert material on which bacteria and other organisms are allowed to flourish 8 P a g e F 2 1 / / M b u c h i E. G a t h o n i

19 on the surface. These organisms oxidise the organic pollutants found in the wastewater as it flows through the filters. This method requires the waste to be sprayed onto the filter and hence requires electricity. Bio-filtration is an aerobic method Tertiary Treatment This refers to any treatment process employed after secondary treatment. It is done before discharging the effluent into the environment or before reusing the water. It provides additional treatment to the water by removing any remaining harmful substances and improves the effluent quality. The common tertiary treatment methods used include; Biological means such as lagoons and maturation ponds Constructed wetlands Chemical addition such as chlorination which kills any pathogens left in the water Ultra-violet radiation Use of filters Below is a flow diagram showing in summary the treatment stages involved. Preliminary Treatment (Removal of floating inorganic solids ) Primary Treatment (Removal of settleable inorganic solids) Secondary Treatment (Removal of organic pollutants) Tertiary Treatment (Purification from remaining organic pollutants) Fig. 2.1: Flowchart of the stages in wastewater treatment. 9 P a g e F 2 1 / / M b u c h i E. G a t h o n i

20 A two- chambered settler and Anaerobic Baffle Reactor (ABR) The treatment starts with a settling chamber which acts as the primary treatment for larger solids and impurities (SASSE 1998). The majority of settleable solids are removed in a settler chamber in front of the actual ABR. Small-scale stand-alone units typically have an integrated settling compartment, but primary sedimentation can also take place in a separate settler for larger systems. There are access covers above each chamber of the settler which allow for sample collection, maintenance and monitoring of the system. An anaerobic baffled reactor (ABR) is an improved Septic Tank with a series of baffles under which the grey, black or industrial wastewater is forced to flow under and over the baffles from the inlet to the outlet. The increased contact time with the active biomass (sludge) results in improved treatment. ABRs use anaerobic bacteria to treat wastewater. Anaerobic treatment offers the following advantages; Less energy requirement as no aeration is needed as compared to kwh for every 1kg of COD removal by aerobic processes. Energy generation in the form of methane gas (1.16kWh energy is produced for every 1kg of COD fermented in anaerobic process). Less biomass (sludge) generation (anaerobic process only 20% of sludge compared with aerobic process). Organic loading rates of 5-10 times higher than that of aerobic processes are possible Saves on space- smaller reactors. Disadvantages of anaerobic treatment; The main disadvantage of anaerobic treatment is that it is not good at removing nonorganic pollution within wastewater, such as nutrients or disease-causing microorganisms (pathogens). All anaerobic processes provide only partial treatment and therefore require the effluent to be treated further before it can be safely released into the environment. 10 P a g e F 2 1 / / M b u c h i E. G a t h o n i

21 To overcome this disadvantage, tertiary treatment is done. However, since the wastewater is not being reused, the disadvantage is not taken as a major drawback. The ABR consists of a tank and alternating hanging and standing baffles that compartmentalise the reactors and force liquid to flow up and down from one compartment to the next, enabling an enhanced contact between the fresh wastewater entering the reactor and the residual sludge, containing the microorganisms responsible for anaerobic digestion of the organic pollutants. The compartmentalised design separates the solids retention time from the hydraulic retention time, making it possible to anaerobically treat wastewater at short retention times of only some hours (EPA 2006). Solids high treatment rates are high, while the overall sludge production is characteristically low (FOXON et al. 2004). They are simple to build and simple to operate, as well as very robust to hydraulic and organic shock loading (SASSE 1998). Typical inflows range from 2 to 200 m3 per day. Critical design parameters include a hydraulic retention time (HRT) between 48 to 72 hours, upflow velocity of the wastewater below 0.6 m/h and the number of upflow chambers (3 to 6). The connection between the chambers can be designed either with vertical pipes or baffles. For this design, vertical pipes are used. Accessibility to all chambers (through access ports) is necessary for maintenance. The reactor always starts with the first chamber for settling out solids (SASSE 1998) followed by a series of at least 2 (MOREL & DIENER 2006), sometimes up to 5 (SASSE 1998) up-flow chambers. The wastewater enters the chambers at the bottom and needs to pass through the sludge to move up and to the next compartment. Thereby particles settle against the up-stream (SASSE 1998). As the wastewater passes through the sludge, intensive contact between the active biomass in the resident sludge and newly incoming wastewater occurs. To equally distribute the entering liquid in the chambers, they should be designed as relatively short compartments (< 75 cm of length and < 50% to 60% of the height, SASSE 1998). To retain any possible scum formed in the up-flow chamber, the outlets of each tank as well as the final outlet should be placed slightly below the liquid surface (SASSE 1998). The up-flow velocity is the most crucial parameter for dimensioning, especially with high hydraulic loading. It should not exceed 2.0 m/h (SASSE 1998; MOREL & DIENER 2006). Based on a given HRT, the up-flow velocity increases in direct relation to the reactor height. Therefore, the reactor height cannot serve as a variable parameter to design the reactor for the 11 P a g e F 2 1 / / M b u c h i E. G a t h o n i

22 required HRT. The limited upstream velocity results in large but shallow tanks. It is for this reason that the baffled reactor is not economical for larger plants (SASSE 1998). The organic load should be below 3 kg COD/m3/day. Higher loading-rates are possible with higher temperature and for easily degradable substrates (SASSE 1998). During the anaerobic digestion, biogas is produced, which can be recovered and reused in the kitchen or for driving pumps and other equipment when necessary. Methane concentration increases steadily from the first compartment to the last (WANG et al. 2004). The methane producing activity of anaerobic sludge in different compartments depends on the substrate, which suggests that the proper anaerobic consortium in each separate compartment develops in accordance to the substrate available and the specific environmental conditions (WANG et al. 2004). The use of the produced biogas in the kitchen might be the most realistic and easiest way to reuse the biogas in decentralised systems. If the gas is not recovered, the tanks need to be vented to prevent the release of the potentially harmful gases (TILLEY et al. 2008). Treatment performance of ABRs is in the range of 65% to 90% COD (Chemical Oxygen Demand) removal, corresponding to about 70% to 95% of BOD (Biological Oxygen Demand) (SASSE 1998; MOREL & DIENER 2006; BORDA 2008). An ABR requires a start-up period of several months to reach full treatment capacity since the slow growing anaerobic biomass first needs to be established in the reactor. To reduce start-up time, the ABR can be inoculated with anaerobic bacteria, e.g., by adding fresh cow dung or septic tank sludge. The added stock of active bacteria can then multiply and adapt to the incoming wastewater. ABRs are generally low cost. However, the costs vary depending on the availability of materials and economy of scale (EAWAG/SANDEC 2008). In any case, ABRs have a high potential to be used in DEWATS. Since they do not require any electricity and are simple to construct and operate, they are generally cheaper than more mechanical, centralised technology options. ABRs can be constructed with locally available material with expert design. Each chamber in the ABR has an access cover above it which allows for monitoring and maintenance of the reactor as well as sample collection if needed. 12 P a g e F 2 1 / / M b u c h i E. G a t h o n i

23 In Kenya, ABRs have been adopted in different areas as a preferred alternative to treat wastewater. Fig. 2.2: Tilley et al 2014 Schematic of the Anaerobic Baffled Reactor. The Anaerobic Baffle Reactor is has been used by Kenyan communities to solve their wastewater problems. Fig. 2.3: ABR Formwork on Ensobosobo Farm in Ngong showing the access covers. 13 P a g e F 2 1 / / M b u c h i E. G a t h o n i

24 Fig. 2.4: ABR construction in Meru Prison Tertiary Treatment Different methods are available for tertiary treatment of wastewater. These are selected based on the quality of effluent needed, space available and availability of raw materials. Biological treatment such as in lagoons and maturation ponds require a large space for implementation. In these methods, it is observed that there is odour production. This makes it unsuitable for areas with close proximity to settlements. They are therefore not preferred for towns and cities. Constructed wetlands/planted gravel filters are an attractive alternative for tertiary treatment. This is because the materials used are locally available and the design is simple. They can also be used for beautification while serving their main purpose. Chemical addition such as chlorination can be used for ensuring no pathogens are left in the water. Chemical addition is mostly used where the raw wastewater had a high concentration of waste and coliforms. It is however not preferred for DEWATS because DEWATS aim to emulate natural processes as much as possible while being as low maintenance as possible. 14 P a g e F 2 1 / / M b u c h i E. G a t h o n i

25 Ultra-violet radiation can also be used to remove the remaining pathogens. It is often combined with lagoons which allow the sun s rays to continuously treat the water through UV rays. Filters can be inform of planted filters (i.e. constructed wetlands) or unplanted filters. Unplanted filters work just as well as planted filters and are easy to implement. The materials needed for their implementation are also easily available locally. Filters were considered for tertiary treatment in the project since they are preferred in DEWATS together with constructed wetlands. They hold the advantage of being cheaper to construct and needing less maintenance than the other alternatives. Filters hold an advantage over constructed wetlands because they require less materials to set up yet they perform the same. Two filters were considered as options; Horizontal flow roughing filters Fig. 2.5: Simple diagram of horizontal flow roughing filter (HFRF). A horizontal flow roughing filter is a system used to filter waster. It consists of differently sized media separated by perforated walls. Water enters the system via a perforated wall and flows horizontally. It is filtered until it gets to the next perforated wall where it flows into the next media. The sizes of the particles reduce with each compartment. 15 P a g e F 2 1 / / M b u c h i E. G a t h o n i

26 Vertical flow roughing filters Fig. 2.6: Simple diagram of vertical flow roughing filters (VFRF). The principal of operation of vertical flow roughing filter is similar to the horizontal, but differs in the direction of flow of the water. It also differs in the construction. In vertical flow roughing filters, the perforation holding the media is at the bottom of each media. Vertical flow roughing filters operate either as down flow or up flow filters. In down flow roughing filters, the water is supplied directly on top of the coarse material. It is filtered as it moves down the material. Once it reaches the bottom, it goes through the perforation holding the filter material and it rises up a tube, flowing onto the second compartment. This process repeats itself up to the final fine material and is finally collected. In up flow roughing filters, water is supplied at the bottom of the filter and passes up through the perforation into the filter medium. When it reaches the top of the media, it flows down a tube, going under the second filter media. This process repeats itself until it goes through the final filter material and is collected. For this project, a horizontal flow roughing filter was used for tertiary treatment. Unlimited filter length and a simple layout are the main advantages of horizontal-flow roughing filters. This allows easy adaptation to meet the needs of the design in question. The structure is shallow and the filter length is not limited to a few metres. Furthermore, its simple layout does not require additional hydraulic structures and installations as in vertical flow roughing filters, thus making it cheaper to install and maintain than the vertical flow roughing filter. The raw 16 P a g e F 2 1 / / M b u c h i E. G a t h o n i

27 water runs in the horizontal direction from the inlet compartment, through a series of differently graded filter material separated by perforated walls, to the filter outlet as illustrated in the figure below. Filter material ranges between 20 and 4 mm in size, and is usually distributed as coarse, medium and fine fraction in three subsequent filter compartments. To prevent algal growth in the filter, the water level is kept below the surface of the filter material by a weir or an effluent pipe placed at the filter outlet. Periodic cleaning is essential for horizontal flow roughing filters. It is carried out by fast drainage of water through the filter. The drainage system is placed at the bottom perpendicular to the direction of flow. Drainage facilities in flow direction must be avoided as they could create short-circuits during normal filter operation. Fig. 2.7: Components of a horizontal flow roughing filter. 17 P a g e F 2 1 / / M b u c h i E. G a t h o n i

28 2.1.3 Flow process Layout The three components of the treatment facility are the settler, the ABR and the Horizontal flow roughing filter. SETTLER ANAEROBIC BAFFLE REACTOR Fig. 2.8: Flow process layout showing the components in the system. 18 P a g e F 2 1 / / M b u c h i E. G a t h o n i

29 2.1.4 Pollutants The parameters used to measure pollution in wastewater are Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD). COD is the most general parameter. It describes how much oxygen is required to oxidise all organic and inorganic matter found in the wastewater sample. BOD describes what can be oxidised biologically, with the help of bacteria and is always a fraction of COD. Usually BOD is measured as BOD5 meaning that it describes the amount of oxygen consumed over a five-day measurement period. It is a direct measurement of the amount of oxygen consumed by organisms removing the organic matter in the waste. SS (Suspended Solids) describes how much of the organic or inorganic matter is not dissolved in water and contains settleable solids that sink to the bottom in a short time and non-settleable suspended solids. All parameters are measured in mg/l. The system designed should therefore be able to remove these pollutants from the wastewater up to an acceptable level set by NEMA before discharge into the environment. 2.2: THEORETICAL FRAMEWORK The following formula and design criteria are used in the design process Quantity of wastewater The total quantity of wastewater is determined from the population to be served by the treatment facility. The current population and the per capita water usage gives the current volume of wastewater produced. With a design period of 20 years, it is possible to predict the volume required after this time. P t = P 0 (1+GR) n...(2.2.1) Where; P t is the population after 20 years P 0 is the current population GR is the population growth rate in decimal n is the design time 19 P a g e F 2 1 / / M b u c h i E. G a t h o n i

30 In designing the ABR for wastewater treatment, we calculate the peak hourly flow in order to obtain the peak daily flow. Peak hourly flow = Peak Factor x Average flow of wastewater per hour....(2.2.2) Peak Factor is calculated using Harmon s equation for peak factor; Peak Factor, PF =.. (2.2.3) Where; P is the population in thousands Peak daily flow = Peak hourly flow x 24hrs Conveyance Line Manning s Equation for sewers flowing under gravity; V = R 2/3 S 1/3... (2.2.4) Where; V is velocity of flow in m/s R is hydraulic mean depth = S is slope of the sewer n is coefficient of roughness of pipes Settler The area required for the settler is calculated using the formula; Area required = 0.5 x Wastewater volume/day..... (2.2.5) Depth of settler =......(2.2.6) Anaerobic Baffle Reactor Volume of the Anaerobic Baffle Reactor The Reactor Working Volume, V w, is the total volume of the ABR given by; V w =...(2.2.7) 20 P a g e F 2 1 / / M b u c h i E. G a t h o n i

31 Where; F is the peak volume of wastewater per day (m 3 /d) HRT is the Hydraulic Retention Time (hours) Peak up flow velocity The peak up-flow velocity in the ABR, v p is the velocity in m/h at which the wastewater flows through the layer of sludge that does not cause an unacceptable entrainment and washout of sludge as it undergoes anaerobic digestion. It should not exceed 2m/h for optimum performance. In this design it is taken as 0.54m/h which is the recommended optimum. Design up flow velocity The design up flow velocity of the ABR, vd is the peak up flow velocity divided by a factor 1.8. This factor was experimentally determined and is set. It is thus given as; vd =....(2.2.8) Where; vd is the design up flow velocity (m/h) vp is the peak up flow velocity (m/h) Compartment up flow area The compartment up flow area, A u, is the area of individual compartments in the ABR. It is given as A u =...(2.2.9) Where; F is the peak volume of flow per day (m 3 /d) 21 P a g e F 2 1 / / M b u c h i E. G a t h o n i

32 vd is the design up flow velocity (m/h) Total compartment area Total compartment area of the ABR, A c, is the total area occupied by all the compartments. It is given by; A c = A u x.. (2.3.0) Where; A u is the compartment up flow area (m 2 ) R U:D is the compartment up flow to down flow area ratio (m 2 /m 2 ) Reactor width The reactor width of the ABR is given by; r w =... (2.3.1) Where; V w is the reactor working volume (m 3 ) C W:L is the compartment width to length ratio N is the number of compartments r D is the reactor depth (m) Reactor length The total length of the reactor is given by; r L =... (2.3.2) Where; 22 P a g e F 2 1 / / M b u c h i E. G a t h o n i

33 N is the number of compartments C W:L is the compartment width to length ratio r w is the width of the reactor (m) Recommended parameter ranges in the design of an ABR Parameter Symbol Unit Recommended Peak Up flow velocity vp m/h 0.54 parameter Number of compartments N - 4 to 6 Hanging baffle clearance dh m 0.15 to 0.20 Up flow to down flow area ratio RU:D m2/m2 2 to 3 Compartment width to length ratio CW:L m/m 3 to 4 Reactor depth Table 2.1: Recommended parameter ranges in ABR design. rd Source: K.M. Foxon and C.A. Buckley m 1 to 3 To maximize the amount of contact time between suspended or dissolved contaminants and the biomass (hence maximizing on the treatment of the effluent), and minimize the amount of sludge washout in the ABR effluent, we; Maximize the HRT Maximize the number of passes through the sludge bed (i.e. number of compartments) Minimise the up flow velocity to reduce solids carry over NB: HRT dictates the size of the reactor (working construction). Peak up flow velocity is the maximum permitted up flow in reactor that does not cause unacceptable washout of sludge. Reactor width to length ratio does not have a direct effect on the superficial up flow velocity 23 P a g e F 2 1 / / M b u c h i E. G a t h o n i

34 2.2.5 Horizontal flow roughing filters Filtration Rate Filtration rate is defined as the hydraulic load (m³/h) per unit of vertical cross section area (m²) of the filter. It ranges from 0.3 to 1.5 m/h. It is given by; V F = = = 0.3 to 1.5m/h (2.3.3) Where; Q is the flow rate into filter (m 3 /h) H is the filter depth (m) between 0.8 to 1.2 m W is the filter width (m) A is the filter cross-sectional area (m 2 ) Drainage velocity Drainage velocity is the velocity necessary to achieve a good hydraulic cleaning efficiency. It ranges between 60 to 90 m/h. It is given by; Vd = = 0.3 to 1.5m/h..(2.3.4) Where; Q d is the drainage rate (m 3 /h) L 1, L 2, L 3 are the lengths of the filter material W is the filter width (m) Efficiency of the filter The efficiency of the filter is calculated using the formula below. E = = e -. (2.3.5) Where; 24 P a g e F 2 1 / / M b u c h i E. G a t h o n i

35 Ce is the concentration of the influent Co is the concentration of the effluent is the change in length i.e. length of filter Removal of Pollutants The expected component efficiencies and percentage pollutant reduction rates were obtained from BORDA studies done on DEWAT systems. These were used to determine the expected effluent characteristics and compare them to NEMA disposal standards. % BOD removal rate =... (2.3.6) Settler Efficiency of 25% to 30% 40% BOD removal 25-50% COD removal 65% Suspended Solids reduction ABR Efficiency of 75% to 85% 70-95% BOD removal, 65-95% COD removal % Suspended Solids reduction (Source: Ludwig, S DEWATS Decentralised Wastewater Treatment in Developing Countries, Bremen Overseas Research and development Association (BORDA), Bremen). 25 P a g e F 2 1 / / M b u c h i E. G a t h o n i

36 3.1 Generation of Concept Design CHAPTER 3: METHODOLOGY Problem Identification Data Collection Assesment of alternative methods Design of Components Performance of System in Pollutant Removal Fig. 3.1: Generation of concept design. 26 P a g e F 2 1 / / M b u c h i E. G a t h o n i

37 3.2 Problem Identification Identification of the problem was done after visiting the site. Residents of Nyari Estate are forced to install their own septic tanks because of the distance of the estate from the nearest main sewer line. This in some cases is more expensive than having a joint treatment plant. Also, since each resident installs their own system, it is hard to monitor the quality of the effluent before it is released to the environment. This poses an environmental problem. 3.3 Site selection Within the estate, there were three potential sites suitable to locate the treatment plant. The three sites were narrowed down to one based on the topography, accessibility and availability of land. 3.4 Data Collection Relevant data was collected which aided in the design process. Data was collected through conducting a desk study and by going to the field Population The population of the residential area was obtained from the number of homes in the area. A design period of 20 years was used to calculate the design flow of the treatment plant with population growth COD, BOD and SS Typical values of COD, BOD and SS concentrations in domestic wastewater were obtained from secondary data. These values were compared to NEMA disposal standards so as to select a suitable treatment method Topography Topography of the area and the slope of the land were observed through a walk over survey and by using Google Earth. 3.5 Assessment of Alternatives Alternative methods of treating the waste were considered. The research was done on the lines of; Advantages and disadvantages of each method. Population served by the plant- This gives the hydraulic and organic load to be handled and hence the most suitable treatment method. 27 P a g e F 2 1 / / M b u c h i E. G a t h o n i

38 Space available- Since the plant will be located in a semi-urban area, the space available will influence the size of the facility. Availability of technology The treatment used should be locally available, applicable and within economic reach. 3.6 Design of components After selecting the Anaerobic Baffle Reactor and Horizontal Flow Roughing Filter as the treatment methods, design of each component was done. The components of the system were sized based on the research done and data collection Primary treatment A two chamber settling tank was selected. The sizing of the settling tank was be calculated based on inflow volume Secondary treatment An Anaerobic Baffle Reactor was selected for secondary treatment. The working volume of the tank, the number of chambers used and the detention time were determined from the inflow volume per day Tertiary treatment A horizontal flow roughing filter was used as the tertiary treatment. The filter dimensions were calculated based on the inflow rate and filtration velocity. 3.7 Performance of the system The performance of the plant was calculated using the initial and final effluent characteristics i.e. BOD, COD and SS. Each component performance was calculated separately. 3.8 Drawings Using the calculated dimensions, engineering drawings of each component were done. 28 P a g e F 2 1 / / M b u c h i E. G a t h o n i

39 CHAPTER 4: RESULTS Data analysis was done based on specific objectives to obtain results. 4.1 Objective 1; Determining the quantity and quality of waste water. QUANTITY OF WASTEWATER 1. Obtain the current population to obtain the amount of wastewater produced 2. Calculate the projected population for 20 years (design time) 3. Calculate the peak flow using the peak factor Total No. of housing units in the estate = 48 Each housing unit contains an average of 5 people Kenya Population Growth Rate (GR) = 2.4% Based on datasheet Current population = No. of housing units x Average size of occupants in one unit = 48 x 5 = 240 people Design Time (D t ) = 20 years Projected population growth for 20 years; P t = P 0 (1+GR) n = 240 ( ) 20 = 385 people which is approximately 400 people Where; P t is the population after 20 years P 0 is the current population GR is the population growth rate in decimal n is the design time 29 P a g e F 2 1 / / M b u c h i E. G a t h o n i

40 Water use per person per day = 150L/p/d = 0.15m 3 /p/d (from water supply design manual page A30) The return amount of wastewater is = 85% Thus, wastewater produced per person per day = x 0.15 =0.1275m 3 /p/d Average wastewater flowrate received (Q ave ) = x 400 people = 51m 3 /d Peak Factor, PF = P = = 0.4 PF = = 4 The value corresponds to WASA Lahore design considerations for sewer design i.e. for average daily flows of 2500, the Peak Factor is 4 Peak hourly flow = 4 x = 8.5m 3 /h Peak daily flow = 8.5 x 24 = 204m 3 /d QUALITY OF WASTEWATER 1. Obtain the typical characteristics of raw sewage (BOD, COD and SS) 2. Compare to NEMA disposal standards to calculate the performance of the system Typical raw wastewater content concentrations are given in the table below DOMESTIC Raw Wastewater Content Typical Concentrations (mg/l) BOD COD 400 SS 200 Table 4.1: Typical pollutant concentrations in raw wastewater. Before treated wastewater is released into the environment, NEMA requires it to be treated to certain standards as shown below. Treated Waste Content NEMA Disposal Standards (mg/l) BOD5 30 COD 50 SS (suspended solids) 30 Table 4.2: NEMA disposal requirements. 30 P a g e F 2 1 / / M b u c h i E. G a t h o n i

41 Source: Johns et al., 1995; Manjunath et al., 2000, NEMA 4.2 Objective 2: Propose a layout of the wastewater conveyance path (along the road network). 1. Generate the elevation profile for the road network 2. Propose the conveyance path for flow by gravity using the elevation profiles 3. Using the manning s equation and a velocity of 0.9m/s (to prevent corrosion or deposition of solids), calculate the slopes of the sewer Fig 4.1: Map showing the road network in Nyari Estate. 31 P a g e F 2 1 / / M b u c h i E. G a t h o n i

42 The elevation profiles for the selected paths are below; Path 1 Path 2 Path 3 32 P a g e F 2 1 / / M b u c h i E. G a t h o n i

43 Path 6 Path 8 Path 9 Path P a g e F 2 1 / / M b u c h i E. G a t h o n i

44 Path 13 Path 14 Path 16 Path P a g e F 2 1 / / M b u c h i E. G a t h o n i

45 Path A Path B Path C Path D Path E 35 P a g e F 2 1 / / M b u c h i E. G a t h o n i

46 Using the elevation profiles, the following path was selected Fig 4.2: Proposed conveyance path layout. Pipes of 225, 450 and 800mm pipes were used Conveyance 1; Manning s Equation for sewers flowing under gravity; V = R 2/3 S 1/3 Selecting a pipe of diameter, D, of 225mm = 0.225m Where; 36 P a g e F 2 1 / / M b u c h i E. G a t h o n i

47 V is velocity of flow in m/s which should be between 0.6m/s and 2.4m/s R is hydraulic mean depth = S is slope of the sewer n is coefficient of roughness of pipes, PVC = Using this formula to calculate the slope of the system while letting V= 0.9m/s V = R 2/3 S 1/3 0.9 = R 2/3 S 1/3 0.9 = ) 2/3 S 1/3 S = Conveyance 2; Calculating the slope of piping system using Manning s equation. Where; V is velocity of flow in m/s which should be between 0.6m/s and 2.4m/s R is hydraulic mean depth = S is slope of the sewer n is coefficient of roughness of pipes, PVC = D of 450mm Using this formula to calculate the slope of the system while letting V= 0.9m/s V = R 2/3 S 1/3 0.9 = R 2/3 S 1/3 0.9 = ) 2/3 S 1/3 S = Conveyance 3; Manning s Equation for sewers flowing under gravity; V = R 2/3 S 1/3 Using this formula to calculate the slope of the system while letting V= 0.9m/s V = R 2/3 S 1/3 37 P a g e F 2 1 / / M b u c h i E. G a t h o n i

48 Where; V is velocity of flow in m/s which is chosen at 0.9m/s R is hydraulic mean depth =, where D is selected as 800mm S is slope of the sewer n is coefficient of roughness of pipes, PVC = = R 2/3 S 1/3 0.9 = ) 2/3 S 1/3 S = Objective 3; Designing the components of the wastewater treatment system (A two compartment settling tank, a six chamber anaerobic baffle reactor and a horizontal flow roughing filter and determine their performance). 1. Calculate the dimensions of the settler (primary treatment) 2. Calculate the dimensions of the ABR (Secondary treatment) 3. Calculate the dimensions of HFRF (Tertiary Treatment) SETTLER Design Criteria; Length to width ratio = 3:1 Depth is between 1 to 2.5m Two chambered settler: First chamber is 2/3 of total length of settler Taking a desludging period of 1 year; Volume of sludge produced per capita per day = 0.1L Total sludge produced per year = 400 x 0.1 x 365 = 14,600L = 14.6 m 3 Dimensions of settler; 38 P a g e F 2 1 / / M b u c h i E. G a t h o n i

49 Area required = 0.5 x Wastewater volume/day = 0.5 x 51m 3 /d = 25.5m 2 With L: W ratio of 3:1 3x*x = 25.5m 2 W= x = 2.92m L= 3 x 2.92 = 8.76m 1 st chamber; Length = x 8.76 = 5.84m 2 nd chamber; Length = =2.92m Depth of settler is based on the retention time. Taking a minimum retention time of 3hrs, Average retention wastewater flow per hour = = 2.123m 3 /h Volume of settler based on minimum retention time = 2.125m 3 /h x 3hrs = 6.375m 3 Total volume of settler = volume of settler + sludge volume = = m 3 Depth of settler = = = 0.82m 1.0m ABR Design Parameters The following six independent parameters are fixed; 1. Design HRT (Hydraulic Retention Time) 2. Number of compartments 3. Peak-up flow velocity 4. Compartment width to length ratio 39 P a g e F 2 1 / / M b u c h i E. G a t h o n i

50 5. Reactor depth 6. Compartment up flow to down flow area ratio Baffle Sizing Using the following recommended parameters (design criteria); 1. Design HRT (Hydraulic Retention Time) =42 hours for start-up 2. Number of compartments, N = 6 3. Peak up flow velocity = V p = 0.54m/h 4. Compartment width to length ratio C W:L = 3 5. Reactor depth, r D = 3m 6. Compartment up flow to down flow area ratio R U:D = 3 7. Hanging baffle allowance =, d h = 0.2 Average Flow, F = 51m 3 /d Recommended Hydraulic Retention Time (HRT) = 42hrs (for startup) 1) Reactor Working Volume, V w = = =357m 3 Where; F is the peak volume of wastewater per day (m 3 /d) HRT is the Hydraulic Retention Time (hours) 2) Design up-flow velocity, vd = Where; = =0.3m/h v d is the design up flow velocity (m/h) v p is the peak up flow velocity (m/h) 3) Compartment up flow area, A u = 40 P a g e F 2 1 / / M b u c h i E. G a t h o n i

51 = = 28.33m 2 Where; F is the peak volume of flow per day (m 3 /d) v d is the design up flow velocity (m/h) 4) Total compartment area = A u x = x =37.77m 2 Where; A u is the compartment up flow area (m 2 ) R U:D is the compartment up flow to down flow area ratio (m 2 /m 2 ) 5) Reactor width, r w = = = 7.71m Where; V w is the reactor working volume (m 3 ) C W:L is the compartment width to length ratio N is the number of compartments r D is the reactor depth (m) 6) Reactor length = r L = = = 15.42m Where; N is the number of compartments C W:L is the compartment width to length ratio 41 P a g e F 2 1 / / M b u c h i E. G a t h o n i

52 r w is the width of the reactor (m) Thickness of walls between baffles = 20cm = 0.2m Thickness of outer walls = 50cm = 0.5m Actual length of each up flow chamber = = 2.32m HORIZONTAL FLOW ROUGHING FILTER Design parameters; Source: Surface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SANDEC - SKAT, 1996, 180 p.: The difference in head between the inflow into the system and the outflow is usually 30cm ( H = 30cm) The sum of the lengths, (i.e. the total length) L 1 + L 2 + L 3 should lie between 5m and 7m L 1 = 2-4m, take as 4m L 2 = 1-3m, take as 2m L 3 = 1-2m, take as 1m Gravel sizes (d g ); d g1 = 12-18mm d g2 = 8-12mm d g3 = 4-8mm Filtration Rate V F = = = 0.3 to 1.5m/h Q is the flow rate (m 3 /h) H is the filter depth (m) between 0.8 to 1.2 m W is the filter width (m) A is the filter cross-sectional area (m 2 ) 42 P a g e F 2 1 / / M b u c h i E. G a t h o n i

53 Letting the filtration rate be 0.3m/h to allow maximum time in the filter Q into filter 51m 3 /d = 2.125m 3 /h 0.3 = = = 0.3 = Therefore, A = 7.08m 2 If selected depth (H) = 1.2m Therefore width, W = 5.9m L= 7m BOD, COD, SS removal Raw Wastewater Content Typical Concentrations (mg/l) BOD COD 400 SS 200 Table 4.3: Typical Wastewater concentrations. Settler Efficiency of 25% to 30% 40% BOD removal 25-50% COD removal 65% Suspended Solids reduction (Source: Ludwig, S DEWATS Decentralised Wastewater Treatment in Developing Countries, Bremen Overseas Research and development Association (BORDA), Bremen). BOD; 40% removal = x 300mg/L = 120mg/L removed Remaining BOD = 300mg/L 120mg/L =180mg/L COD; 30% removal = x 400mg/L = 120mg/L 43 P a g e F 2 1 / / M b u c h i E. G a t h o n i

54 Remaining COD = 400mg/L 120mg/L = 280mg/L SS 65% removal = x 200mg/L =130mg/L Remaining SS = 200mg/L 130mg/L = 70mg/L ABR Efficiency of 75% to 85% 70-95% BOD removal, 65-95% COD removal % Suspended Solids reduction (Source: Ludwig, S DEWATS Decentralised Wastewater Treatment in Developing Countries, Bremen Overseas Research and development Association (BORDA), Bremen). BOD; 90% = x 180mg/L = 162mg/L Remaining BOD = 180mg/L 162mg/L = 18mg/L COD; 80% removal = x 280 = 252mg/L Remaining COD =280mg/L 252mg/L = 28mg/L SS; 90% removal = x 70mg/L = 63mg/L Remaining SS = 70mg/L 63mg/L = 7mg/L Most of the treatment happens in the ABR and an efficiency of 70-95% is acceptable. Assuming a worst case scenario where no BOD is removed in the settler, therefore BOD into the ABR is 300mg/L (total). Confirming the efficiency of the calculated ABR volume; % BOD Removal rate = 44 P a g e F 2 1 / / M b u c h i E. G a t h o n i

55 BOD eff = BOD in BOD in = 300mg/L K t = K 20 X (Tave-20) X=1.06 T ave = 19.4 K 20 = 1.1d -1 BOD eff = 300e -1.06x1.75 = 46.75mg/L % BOD Removal rate = =0.844 = 84.4% so okay. Organic Loading Rate (OLR) for ABR; OLR = = =0.29kgCOD/m 3 d Value should not exceed 3kgCOD/m 3 d so okay. FILTER Suspended Solids removal efficiency 79-88% (Int. J. of Environment and Waste Management, 2008 Vol.2, No.3, pp ) The filter removes the remaining SS. Assuming a worst case scenario where no SS are removed in the settler and ABR, SS removal is; SS c = SS o [ (HLR)] Where; SS c is the effluent Suspended solids SS o is the SS in inflow HLR is hydraulic loading rate HLR = 45 P a g e F 2 1 / / M b u c h i E. G a t h o n i

56 Where; Q is the flow rate As is the cross-sectional area of filter HLR = = = SS = 200[ (28.81)] = 27.4mg/L This is less than the NEMA disposal standard so okay. Efficiency thus is = = = = 86.34% so okay. If careful implementation of the project is followed, it can be observed that the pollutant removal would satisfactorily meet the NEMA disposal standards as indicated below Treated Waste Content NEMA Disposal Standards (mg/l) BOD5 30 SS (suspended solids) 30 Table 4.4: NEMA disposal standards. 46 P a g e F 2 1 / / M b u c h i E. G a t h o n i

57 4.4 TABLE OF RESULTS (SUMMARY) PARAMETER Method Result QUANTITY OF WASTEWATER Quantity of wastewater (daily) Peak Daily Wastewater Population x (85% of daily water consumption per capita) PF x Daily wastewater PF= PF = 51m 3 /d 204m 3 /d CONVEYANCE PATH Slope of conveyance 1 (225mm Diameter) Slope of conveyance 2 (450mm Diameter) Slope of conveyance 3 (800mm Diameter) V = R 2/3 S 1/ V = R 2/3 S 1/ V = R 2/3 S 1/ SETTLER Sludge produced per year Sludge per capita per day(0.1l) x 14.6 m x Population (400) Area required 0.5 x Wastewater volume/day 25.5m 2 1 st chamber x Total length 5.84m 2 nd chamber x Total length 2.92m Total volume of settler [Minimum retention time (3hrs) x m 3 Average wastewater flow per hour] + sludge volume Depth 1.0m ABR Reactor Working Volume, V w Design up-flow velocity, vd Compartment up flow area, A u 357m 3 = 0.3m/h 28.33m 2 47 P a g e F 2 1 / / M b u c h i E. G a t h o n i

58 Total compartment area A u x 37.77m 2 Reactor width, r w 7.71m Reactor length, r L 15.42m Actual length of each up 2.32m flow chamber HFRF(Horizontal Flow Roughing Filter) Filtration Flow, V F = 0.3 to 1.5m/h A = 7.08m 2 Height 1.2m Width 5.9m Length 7m ABR efficiency 84.4% BOD eff = BOD in K t = K 20 X (Tave-20) Filter efficiency 86.34% SS c = SS o [ (HLR)] HLR = 48 P a g e F 2 1 / / M b u c h i E. G a t h o n i

59 CHAPTER 5: DISCUSSION For the DEWAT system, water demand data was used to calculate the flow rate using 85% of the total demand as the water received into the plant. For a daily flow of 51m3, a peak flow of 204m3/d was calculated using Harmon s equation and used as the design flow which caters for extreme events. A design time of 20 years was used. A conveyance path layout for wastewater was proposed. The velocity at which the waste should flow so as to prevent excessive corrosion or solid deposition lies between 0.6m/s to 2.4m/s. using a velocity of 0.9m/s, the slopes of the sewer pipes were calculated. Elevation profiles were generated along the roads and from these, a suitable path was chosen. For house sewers, a pipe diameter of 225mm was selected. For lateral sewers, a pipe diameter of 450mm was selected. Finally for the sub-main sewer, a diameter of 800mm was selected. From the sewer line, the wastewater goes into a two-chamber settling tank which precedes an anaerobic baffle reactor. This is where primary treatment takes place. The purpose of the settler is to remove the majority of settleable solids by gravity. The first chamber has a length of 2/3 of the total chamber length. With a length to width ratio of 3:1, a depth of 1m was calculated for a minimum retention time of 3 hours. The width was calculated as 2.92m and total length was found to be 8.76m. The total volume of the settler was found to be m3 including the sludge volume expected for a desludging period of 1 year. After leaving the settler, the waste enters the anaerobic baffle reactor where secondary treatment takes place through anaerobic reaction. A hydraulic retention time of 42 hours was selected for a startup plant as recommended. Six chambers were used in the design. A recommended peak up flow velocity of 0.54m/h was used. From this, a design up-flow velocity of 0.3m/h was calculated. The compartment width to length ratio of 3 was selected from a range of 3 to 4. The depth range is 1 to 3m and was set at 3m. Up flow to down flow area ratio which ranges from 2 to 3 was chosen as 3. Reactor volume was calculated and found to be 357m3. For each compartment, area was found to be 37.77m2. Width of the ABR was calculated as 7.71m while length was found to be 15.42m. Most of the biological pollutants are removed during secondary treatment. To calculate the efficiency of the ABR, % BOD removal rate was used. Efficiency was calculated for an extreme event such that no biological pollutant was removed in the settler. 49 P a g e F 2 1 / / M b u c h i E. G a t h o n i

60 BOD reduced from 300mg/L to 46.75mg/L making the removal rate to be 84.4%. This is acceptable since ABR efficiency should range between 70-95%. Tertiary treatment took place in a horizontal flow roughing filter. The filtration rate as set in design standards lies between 0.3m/h to 1.5m/h. In this design, the rate was set at 0.3m/h which gave a filtration area of 7.08m2. The total length of the filter was selected to be 7m with a width of 5.9m and depth of 1.2m. Efficiency of the filter was calculated based on the extreme event that no Suspended Solids were removed in the primary and secondary stages. Using the Hydraulic Loading Rate, the efficiency was found to be 86.34%. Filter efficiency should range between 79-88% therefore making it acceptable. 50 P a g e F 2 1 / / M b u c h i E. G a t h o n i

61 CHAPTER 6: CONCLUSION The quantity and quality of the wastewater produced in the estate were determined. The quantity was determined from a design period of 20 years based on population. The quality was obtained from typical wastewater characteristics. The slopes of the area were determined from Google Earth. This enabled the determination of the slope to be followed for conveying the wastewater and hence the most suitable site. The quantitative and qualitative analysis assisted in selecting the most appropriate treatment method to be used. Components of the system were then designed. The first component designed was a two chambered settler. This is where most of the solids in the wastewater are removed to make the system more effective and operates on the force of gravity acting on the solids. An anaerobic baffle reactor was the next component to be designed. This is where most of the treatment takes place biologically as the wastewater is forced through organic matter (sludge). The treatment takes place anaerobically (in the absence of oxygen). Tertiary treatment takes place in a horizontal flow roughing filter. It operates on filter media separated by perforated walls. Each segment is filled with differently sized media. The media reduces as the water moves from the inlet to outlet. It serves to remove any remaining sediments in the effluent. The overall objective was met. 51 P a g e F 2 1 / / M b u c h i E. G a t h o n i

62 CHAPTER 7: RECOMMENDATIONS The sludge can undergo treatment and drying to make fertilizer which can generate income for the estate residents. The methane produced from anaerobic digestion in the ABR can be collected and used as energy. Introduction of an advanced treatment stage, for example, a stabilization pond would increase the effluent quality. The treated effluent can be used to irrigate lawns. This can be done through sub-surface irrigation hence, evergreen lawns. Application of treated wastewater for irrigation has become a common practice worldwide and a centre of attention to scientists and technologists in developing countries (Hamoda et al., 2004; Lubello et al., 2004). Sampling of the wastewater as it goes through the treatment should be done regularly to ensure the system performs as expected. 52 P a g e F 2 1 / / M b u c h i E. G a t h o n i

63 CHAPTER 8: REFERENCES Syed R. Qasim Waste water treatment plant Planning, Design, and Operation ", Benefield, L.D., And C.W. Randall, "Biological Process Design Of Waste Water Treatment", Prentice - Hall, Englewood Cliffs, N.J Metcalf and Eddy, 1991 Wastewater Engineering - Treatment, Disposal, and Reuse McGraw-Hill, Inc., New York. U.S. EPA, 1993 Combined Sewer Overflow Control Manual EPA-625R Gavle, Darrel R., and David G. Mitchell, 1995 Innovative and Economical SSO Treatment Utilizing Fine Screens and Chlorination Presented at the EPA National Conference on Combined Sewer Overflows, Washington, D.C. Hartmann, L., (1999), Historical Development of Wastewater Treatment Processes, In: J. Winter (Ed), Environmental Processes I, wastewater treatment. Weinheim: WILEYVCH. Couture, M., J. Lamontagne, and B. Gagne, John Meunier, Inc.; O. Dalkir, Cegeo Technologies; and C. Marche, University of Montreal; Abstract of a presentation at the New York Water Environment Association, New York, NY DEWATS A Practical Guide (BORDA, 2009) Khan, A.R Appropriate Wastewater Treatment Processes for N-WFP, Pakistan, Master of Science Research of Loughborough University of Technology Ludwig, S DEWATS Decentralised Wastewater Treatment in Developing Countries, Bremen Overseas Research and development Association, Bremen Mann, H.T., Williamson, D., Water Treatment and Sanitation, Intermediate Technology Publications 1973, 1979, 1982., Printed in England by The Russell Press Ltd., Nottingham. 53 P a g e F 2 1 / / M b u c h i E. G a t h o n i

64 Mara, D. Sewage Treatment in Hot Climates, A Wiley Interscience Publication, John Wiley and Sons. Neptune Pacific Ltd., On-site and Small Community Sewage Management with the N-DN Biofilter Treatment Plant Foxon K, Dama P, Brouckaert C And Buckley C (2001) Design considerations for the implementation of an anaerobic baffled reactor in low-income settlements in Kwa-Zulu Natal. Proc. of the IWA Conf. on Water and Wastewater Management for Developing Countries, Kuala Lumpur, Malaysia. BARBER WP and Stuckey DC (1999) The used of an anaerobic baffled reactor (ABR) for wastewater treatment: A Review. Water Res. 33 (Orozco, 1997) 1559 BELL J and BUCKLEY C (2003) Characterisation of the methanogenic populations in an operating anaerobic baffled reactor. The International Water Association Conference on Environmental Biotechnology, Advancement on Water and Wastewater: Applications in the Tropics, Universiti Teknologi Malaysia, Johor Bahru, Malaysia OROZCO A (1997) Pilot and full-scale anaerobic treatment of lowstrength wastewater at sub-optimal temperature (15ºC) with a hybrid plug-flow reactor. Proc. of the 8th Int. Conf. on Anaerobic Digestion, Sendai, Japan WU W et al. (1985) Properties of granular sludge in upflow anaerobic sludge blanket (USAB) and its formation. In: Anaerobic Digestion, China State Biogas Association, Guangzhou, China. 339 Burian, S. J., Nix, S. J., Pitt, R. E., Durrans, S. R., (2000), Urban Wastewater Management in the United States: Past, Present, and Future, Journal of Urban Technology, 7 (3), Crites, R., Tchobanoglous, G., (1998), Small and Decentralised Wastewater Management Systems, Boston: McGraw-Hill. Jackson, H. B., (1996), Global Needs and Developments in Urban Sanitation, in: D. Mara (Ed.), Low-Cost Sewerage, Chichester: John Wiley & Sons. 54 P a g e F 2 1 / / M b u c h i E. G a t h o n i

65 P a g e F 2 1 / / M b u c h i E. G a t h o n i

66 CHAPTER 9: APPENDICES 9.1 APPENDIX 1: DESIGN OF FILTERS Table Design information for different filter rates. Source: Metcalf & Eddy's "Wastewater Engineering Treatment, Disposal and Reuse 56 P a g e F 2 1 / / M b u c h i E. G a t h o n i

67 Image Filter material that can be used in horizontal flow roughing filtration. Source: CAWST, 57 P a g e F 2 1 / / M b u c h i E. G a t h o n i

68 Graph Turbidity reduction along a roughing filter. Source: Surface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SANDEC - SKAT, 1996, 180 p.) 58 P a g e F 2 1 / / M b u c h i E. G a t h o n i

69 Table Roughing Filter Efficiency in Correlation to Flow Conditions. Source: Surface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SANDEC - SKAT, 1996, 180 p.) 59 P a g e F 2 1 / / M b u c h i E. G a t h o n i

70 Table Recommended guidelines for the design of horizontal flow roughing filters for different qualities of water. Source: S. Afr. j. sci. vol.106 n Pretoria Nov./Dec P a g e F 2 1 / / M b u c h i E. G a t h o n i

71 9.2 APPENDIX 2: TABLE OF MANNING S ROUGHNESS COEFFICIENT n, FOR DIFFERENT MATERIALS Surface Material Manning's Roughness Coefficient - n - Asbestos cement Asphalt Brass Brick Canvas Cast-iron, new Clay tile Concrete - steel forms Concrete (Cement) - finished Concrete - wooden forms Concrete - centrifugally spun Copper Corrugated metal Earth, smooth Earth channel - clean Earth channel - gravelly Earth channel - weedy Earth channel - stony, cobbles Floodplains - pasture, farmland Floodplains - light brush P a g e F 2 1 / / M b u c h i E. G a t h o n i

72 Surface Material Manning's Roughness Coefficient - n - Floodplains - heavy brush Floodplains - trees 0.15 Galvanized iron Glass Gravel, firm Lead Masonry Metal - corrugated Natural streams - clean and straight Natural streams - major rivers Natural streams - sluggish with deep pools Natural channels, very poor condition Plastic Polyethylene PE - Corrugated with smooth inner walls Polyethylene PE - Corrugated with corrugated inner walls Polyvinyl Chloride PVC - with smooth inner walls Rubble Masonry Steel - Coal-tar enamel Steel - smooth Steel - New unlined Steel - Riveted P a g e F 2 1 / / M b u c h i E. G a t h o n i

73 Surface Material Manning's Roughness Coefficient - n - Vitrified Sewer Wood - planed Wood - unplaned Wood stove pipe, small diameter Wood stove pipe, large diameter Source: 63 P a g e F 2 1 / / M b u c h i E. G a t h o n i

74 9.3 APPENDIX 3: WASA LAHORE PEAK FACTOR CHART FOR SEWAGE DESIGN Source: 64 P a g e F 2 1 / / M b u c h i E. G a t h o n i

75 9.4 APPENDIX 4: A GRAPH OF BOD AND COD REMOVAL EFFICIENCY WITH HYDRAULIC RETENTION TIME Source: International Journal of Environmental Health Engineering P a g e F 2 1 / / M b u c h i E. G a t h o n i

76 9.5 APPENDIX 5: FLOW HYDROGRAPH OF WASTEWATER PRODUCTION OVER THE DAY Source: 66 P a g e F 2 1 / / M b u c h i E. G a t h o n i

77 9.6 APPENDIX 6: ELEVATION PROFILES (GOOGLE EARTH) Path 1 Path 2 Path 3 Path 4 67 P a g e F 2 1 / / M b u c h i E. G a t h o n i

78 Path 5 Path 6 Path 7 Path 8 68 P a g e F 2 1 / / M b u c h i E. G a t h o n i

79 Path 9 Path 10 Path 11 Path P a g e F 2 1 / / M b u c h i E. G a t h o n i

80 Path 13 Path 14 Path 15 Path P a g e F 2 1 / / M b u c h i E. G a t h o n i

81 Path P a g e F 2 1 / / M b u c h i E. G a t h o n i

82 Fig: 4.3: Cross-section of two chambered settler 72 P a g e F 2 1 / / M b u c h i E. G a t h o n i

83 Fig 4.4: Plan view of two chambered settler 73 P a g e F 2 1 / / M b u c h i E. G a t h o n i

84 Fig 4.5: Cross-section of ABR 74 P a g e F 2 1 / / M b u c h i E. G a t h o n i

85 Fig: 4.6: Plan view of ABR 75 P a g e F 2 1 / / M b u c h i E. G a t h o n i

86 Fig 4.7: 3D diagram of Horizontal Flow Roughing Filter (HFRF) 76 P a g e F 2 1 / / M b u c h i E. G a t h o n i