Study Session 6 Liquid Waste Management and Treatment

Transcription

1 Study Session 6 Liquid Waste Management and Treatment Copyright 2016 The Open University

2 Contents Introduction 3 Learning Outcomes for Study Session Faeca sudge management Pit emptying Vacuum trucks Vacutug Hand-operated pumps Disposa of the sudge Septic tanks Wastewater treatment options Waste stabiisation ponds Reed beds Mechanica-bioogica wastewater treatment Suage management Stormwater management 18 Summary of Study Session 6 20 Sef-Assessment Questions (SAQs) for Study Session of 24 Wednesday 30 March 2016

3 Introduction Introduction Deaing with iquid wastes is one of the biggest chaenges of urban sanitation. Liquid waste management requires capita investment, skied personne, coordination between different government departments and organisations, and awareness of the issues by the pubic. Faiure to manage iquid wastes, especiay human excreta, eads to heath and environmenta probems. In urban Ethiopia the vast majority of peope (80%) use pit atrines for human waste disposa (Centra Statistica Agency, 2014). In some parts of Addis Ababa, iquid waste from waterborne toiet systems is coected via a sewerage system and sent to the wastewater treatment faciity at Kaity. There are, however, many houses and institutions with waterborne toiet systems that are not connected to the sewerage system. In these paces, the sewage is sent to septic tanks. These different sanitation technoogies require different techniques for managing and disposing of the wastes. This study session ooks at what happens to human excreta and other iquid wastes after they are produced. You wi earn about the methods used to remove sudge from pits and septic tanks and about some of the options for managing and treating iquid wastes, incuding human excreta, suage and stormwater. Learning Outcomes for Study Session 6 When you have studied this session, you shoud be abe to: 6.1 Define and use correcty a of the key words printed in bod. (SAQ 6.1) 6.2 Describe some options for the coection and transport of faeca sudge. (SAQ 6.2) 6.3 List the sources of faeca sudge, and how it can be treated and reused. (SAQ 6.3) 6.4 Describe the main methods for wastewater treatment and expain the differences between them. (SAQ 6.4) 6.5 Suggest a suitabe means for suage disposa. (SAQ 6.5) 6.6 Suggest appropriate methods for stormwater management. (SAQ 6.6) 6.1 Faeca sudge management When human excreta coects in a pit atrine, the soids sette at the bottom and form a surry caed faeca sudge. Over time the sudge accumuates and periodicay needs to be removed and disposed of. This process presents severa chaenges because the sudge is offensive, a potentia danger to human heath and highy pouting if dumped indiscriminatey into the environment. Faeca sudge management (FSM) is a set of processes designed to ensure that peope and the environment are protected from these hazards. It incudes the storage, coection, transport, treatment and safe end use or disposa of faeca sudge (Strande et a., 2014). FSM is a significant probem in towns and cities in many deveoping countries. Key issues are who is responsibe for coecting sudge and where and how it is disposed of. 3 of 24 Wednesday 30 March 2016

4 6.2 Pit emptying The next section describes some of the methods for removing faeca sudge from pit atrines. These methods aso appy to sudge remova from septic tanks, which are described in detai in Section Pit emptying The process of pit emptying is sometimes caed desudging. There are manua and mechanica methods for desudging, but the manua remova of faeca sudge from pit atrines poses severe risks to those undertaking the task. What are the risks associated with manua emptying of pit atrines using buckets and shoves? Firsty, there is risk of contact with faeces which coud contain pathogens and cause disease. There coud aso be a danger of the pit coapsing if someone has to get inside to dig out the sudge. Whether using manua or mechanica methods, the persona safety of anyone empoyed in pit emptying shoud be of primary importance. Operatives shoud wear goves, masks and protective cothing Vacuum trucks Vacuum trucks are vehices equipped with a storage tank and pump with a hose that is owered into the pit to suck the sudge up and out into the storage tank (Figure 6.1). The sudge can then be easiy transported to a suitabe disposa site. Vacuum trucks are quick, powerfu and efficient, but they are arge vehices and so access to the pits can be a probem. The size of the truck can imit their use in areas where roads are narrow and twisting. Truck operators wi charge a fee for their services, which is another factor that needs to be considered. 4 of 24 Wednesday 30 March 2016

5 6.2 Pit emptying Figure 6.1 Vacuum trucks are used to empty pit atrines and septic tanks Vacutug The Vacutug, shown in Figure 6.2, is basicay a smaer version of the vacuum truck. It was devised by UN-Habitat as a system that coud repace manua emptying. The Vacutug is a mechanica system that can be manufactured ocay using readiy avaiabe components. It is affordabe, easiy serviceabe, abe to operate in narrow passageways where vacuum trucks cannot go and is capabe of sucking out waste sudge for transportation to a arger tanker vehice (UN-Habitat, 2003). It can empty pits down to 2 m deep (Thye et a., 2009). Figure 6.2 The Vacutug. 5 of 24 Wednesday 30 March 2016

6 6.2 Pit emptying Hand-operated pumps Even the Vacutug cannot get everywhere. The Sudge Guper, ike the one shown in Figure 6.3, is an exampe of a smaer hand-operated pump. These can be taken to pit atrines that are inaccessibe to arger pumps. It is a simpe design consisting of a PVC pipe containing two vaves and can be buit using ocay avaiabe materias. The sudge is pumped up by hand, coected in a container and taken away for disposa. Care is needed to ensure that the operator and other hepers do not come into contact with the sudge and that it is not spied. Figure 6.3 Hand-operated sudge pump. The pump is ocated over the pit and the operator (wearing suitabe protective cothing) pumps the hande up and down. Sudge rises up the vertica pipe and is expeed into the container. (Tiey et a., 2014) Disposa of the sudge Severa options are avaiabe for disposa of the coected sudge (Pickford and Shaw, 2005). It can be put directy onto and and used as a soi conditioner, but this is ony possibe if it has been eft untouched for at east two years (Brikke and Bredero, 2003). Fresh, untreated wet sudge poses high risks for human heath and so shoud not be put on and used to grow crops. Drying the sudge wi ki most pathogens. This can be achieved using drying beds (Figure 6.4), where sudge is put into shaow tanks to a depth of about 300 mm. The base of the tank is soped and covered with a ayer of sand (forming a bed ) to aow iquid to drain out of the sudge. In the warm cimate of Ethiopia and without rain, after about a week the sudge wi be dry enough to be ifted by a shove. 6 of 24 Wednesday 30 March 2016

7 6.3 Septic tanks Figure 6.4 Sudge in drying beds. The sudge can aso be composted by mixing it with vegetabe matter, or biogas can be obtained by anaerobic digestion. Whichever method is used, faeca sudge disposa must be carefuy managed and operated in order to ensure that the associated risks to heath and the environment are avoided. 6.3 Septic tanks A septic tank (Figure 6.5) is an underground and watertight tank made of concrete, fibregass or PVC in which sewage is coected and partiay treated. These are used for water-fushed systems that are not connected to a sewer. Concrete septic tanks are commony used in Ethiopia. What is sewage? Sewage is combined wastewater from a sources and incudes backwater (human excreta), greywater and stormwater. 7 of 24 Wednesday 30 March 2016

8 6.3 Septic tanks Figure 6.5 Diagram of a septic tank. Wastewater enters the tank, remains there for a time, and is dispaced out of the tank by new wastewater coming in. There are no pumps or mechanica parts. The time the wastewater remains in the tank is caed the retention time and shoud be a minimum of 24 hours. In this period, the soid matter in the sewage settes to the bottom of the tank, where it is partiay degraded by anaerobic micro-organisms. The iquid above the sudge is reativey free of soids, but it does contain dissoved organic and inorganic chemicas that are not treated. Light substances such as oi and grease form a scum and foat to the top. The position of the outet (seen in Figure 6.5) ensures that ony water from the midde of the tank is dispaced outwards. This effuent is disposed of to a soakaway (aso caed a soak pit or seepage pit, Figure 6.6) or to a drainfied (Figure 6.7). 8 of 24 Wednesday 30 March 2016

9 6.3 Septic tanks Figure 6.6 Diagram of a soakaway. The soakaway is a covered, unseaed pit ined with bricks or stone. The wastewater from the septic tank seeps into the soi through the base of the pit and through the spaces in the ining materia. A drainfied (Figure 6.7) is a fied where the effuent from a septic tank is disposed of. It can be constructed by aying a network of perforated pipes (pipes with tiny hoes) in excavated soi which has a bed of grave. Soi is paced over the top of the perforated pipe, and pants are grown on top of the drainfied. The wastewater is sent into the perforated pipe and percoates (seeps) through the grave and goes into the ground, repenishing the groundwater. Figure 6.7 Diagram showing the ocation of a septic tank and a drainfied. 9 of 24 Wednesday 30 March 2016

10 6.4 Wastewater treatment options Septic tanks need to be desudged when the sudge depth is approximatey two-thirds the depth of the tank. You can estimate this by putting a graduated stick into the tank. Depending on usage, this is typicay once every 12 months. A vacuum truck is usuay used to suck the sudge out for disposa (Figure 6.8). Figure 6.8 Diagram of a septic tank being desudged by vacuum truck. If a sewer network is ater instaed in an area where there are septic tanks (as often happens in deveoping countries), the sewage fow can be connected directy to the sewer, thus bypassing the septic tank. This aows for better treatment (and possiby reuse) of the sewage, and of course eiminates the need for a septic tank. 6.4 Wastewater treatment options Septic tanks remove soids from sewage, but treatment is minima and the effuent contains high eves of dissoved organic matter and ammonia. Other options for better treatment are avaiabe. The aim in wastewater treatment is to: reduce the amount of biodegradabe materia and soids remove toxic materias eiminate pathogenic micro-organisms. There are severa ways of treating sewage and other wastewaters with different costs and requirements. One that is particuary effective and economica in warm regions of the word is waste stabiisation ponds. 10 of 24 Wednesday 30 March 2016

11 6.4 Wastewater treatment options Waste stabiisation ponds Waste stabiisation ponds are natura or constructed ponds used for treating sewage or other wastewaters bioogicay by harnessing the power of sunight and wind. They are idea for tropica countries ike Ethiopia. The sewage coected by the sewerage network in Addis Ababa goes to waste stabiisation ponds at Kaity. In a typica waste stabiisation pond system, effuent that has passed through a screen is sent through a series of ponds (Figure 6.9) with a tota retention time of between 10 and 50 days. Bacteria in the ponds degrade the organic waste and work symbioticay with agae, which provide oxygen through photosynthesis. (Symbioticay means in a reationship where two types of iving organism ive together for their mutua benefit.) Oxygenation aso occurs through the action of wind and by diffusion from the air. No mechanica equipment is used in the ponds, so operation and maintenance costs are very ow. Land requirement is, however, high. 11 of 24 Wednesday 30 March 2016

12 6.4 Wastewater treatment options Figure 6.9 Layout of a waste stabiisation pond system treating domestic wastewater. The major part of the biodegradation of the sewage takes pace in the facutative ponds. Facutative ponds are m deep, with a retention time of between 5 and 30 days. In these ponds the upper ayers are aerobic and the ower ayers of water are anaerobic. Soids sette to the bottom and are anaerobicay digested, so sudge remova is rarey needed. Maturation ponds are paced after facutative ponds for the purpose of pathogen reduction (Figure 6.10). These are usuay m deep with a retention time of 12 of 24 Wednesday 30 March 2016

13 6.4 Wastewater treatment options between 15 and 20 days. These ponds serve to inactivate pathogenic bacteria and viruses through the action of UV radiation from sunight and the greater aga activity in these shaow ponds, which raises the ph above 8.5. (ph is a measure of acidity and akainity. It has a scae from 0 14: ph 7 is neutra, ess than 7 is acid and more than 7 is akaine.) The ong retention time in the maturation ponds aso enhances the sedimentation of the eggs of intestina parasitic worms. Figure 6.10 A waste stabiisation pond at Mekee University note the green coour of the pond due to agae. If the wastewater has a very high eve of poutants, anaerobic ponds can be used ahead of the facutative ponds. Anaerobic ponds are 2 5 m deep and are neary devoid of oxygen. Their retention time is one to seven days. Soids sette to the bottom, forming a sudge, and anaerobic digestion takes pace, producing methane. Up to 60% of the organic materia can be removed in this process (Tiey et a., 2014). To prevent sewage from eaching away, and to preserve the effuent for reuse ater, the ponds shoud have a iner. This can be made of cay, asphat, compacted earth or any other impermeabe materia (materia that does not et fuid pass through). To prevent run-off from entering the ponds and to prevent erosion, a protective raised earth barrier can be constructed around the ponds using the excavated materia from their construction. Finay, a fence is needed to keep peope and animas out (Tiey et a., 2014). Any scum that buids up on the surface of the facutative and maturation ponds shoud be removed to aow sunight to reach a the agae and aso to increase surface aeration. Large pants that are present in the water shoud be removed. Treated sewage can be reused in crop irrigation if safe imits of faeca bacteria and intestina parasite eggs are achieved in the treatment process. At the same time as treating wastewater, pond systems have been used to increase protein production through the rearing of fish (such as Tiapia) and ducks in maturation ponds Reed beds Reed beds, or artificiay constructed wetands with emergent pants, have been used to treat sewage in many parts of the word. In Ethiopia, studies have been carried out on this type of wastewater treatment system for domestic wastewaters in Addis Ababa 13 of 24 Wednesday 30 March 2016

14 6.4 Wastewater treatment options (Genet, 2007) and Kombocha (Sahu and Yimer, 2014), and brewery effuent in Addis Ababa (Angassa, 2011). Reed beds are idea for warm countries where pants grow rapidy. They have ow operationa costs but they do require a ot of and. The pants (usuay reed species Phragmites austrais or Phragmites communis) are grown in rows in beds of soi or grave ined with an impermeabe cay or synthetic iner (Figure 6.11). The effuent requiring treatment is fed into the bed, which typicay has a depth of 600 mm. The base of the reed bed has a sope to enabe coection of the effuent after treatment. Figure 6.11 The reed bed system. When fu-grown, the reeds are between 1.5 and 3.0 m high. The effuent is distributed through pipes and nozzes onto the reed bed and then percoates down to the roots and rhizomes (horizonta underground stems) of the reeds. The root and rhizome system provides a mix of aerobic and anaerobic conditions that encourage a diversity of microbia species in the soi. As a resut, the reed bed system has potentia for treating a wide range of poutants. For exampe, athough micro-organisms that are capabe of biodegrading many synthetic chemicas (such as some common pesticides) are found in soi, they not normay present in effuent treatment pants, so reed beds can be effective for treating effuents that contain these types of chemicas. Can you think of an additiona advantage that reed beds offer? Reed beds are very attractive to birds and thus increase the diversity of widife where they are constructed Mechanica-bioogica wastewater treatment Wastewater treatment can be undertaken using a sequence of processes in a mechanica-bioogica system (Figure 6.12). This treatment is faster than using natura systems such as waste stabiisation ponds or reed beds and requires ess space. These factors make it desirabe for sewage treatment in towns with arge popuations where 14 of 24 Wednesday 30 March 2016

15 6.4 Wastewater treatment options there is not enough and for natura systems. However, mechanica-bioogica systems are more expensive because of the equipment required and the need for skied personne to operate them. Figure 6.12 A mechanica-bioogica wastewater treatment pant. These systems typicay have three main stages: preiminary, primary and secondary treatment. Preiminary treatment In this first stage screens (Figure 6.13) remove arge items such as pieces of wood, meta, rags, paper or pastic that have got into the sewerage system. Removing them protects the structures and equipment in the wastewater treatment pant. Paper and rags in the wastewater fow are sometimes shredded by comminutors, which are rotating, sotted drums equipped with cutting bades. The shredded materia can then be returned to the fow further downstream without causing harm. 15 of 24 Wednesday 30 March 2016

16 6.4 Wastewater treatment options Figure 6.13 Screening of wastewater. Soid materias are retained on the screen whie the iquid passes through. Sma stones and grit have to be removed from the fow. Grit is comprised of very sma pieces of sand, stone and possiby aso gass and meta. A these materias can increase the rate of wear of mechanica equipment and can aso sette easiy in pipes, causing bockages. The grit settes out in grit channes (Figure 6.14) and can be removed daiy by manua or mechanica means and dumped at a andfi site. Figure 6.14 gravity. A grit channe that sows the rate of fow and aows grit to sette out by 16 of 24 Wednesday 30 March 2016

17 6.5 Suage management Primary treatment In the primary treatment stage, fine soids in the wastewater are removed by settement in a sedimentation tank (Figure 6.15). A propery designed and we-operated primary sedimentation tank wi reduce the suspended soids content of the wastewater by between 50 and 70%, and the biochemica oxygen demand (BOD) by between 25 and 40% (Crites and Tchobanogous, 1998). Figure 6.15 Diagram of a rectanguar sedimentation tank. Secondary treatment This is the bioogica stage of treatment. In secondary treatment the organic matter in the sewage is biodegraded by micro-organisms using oxygen. Oxygen eves are increased artificiay by various means to ensure remova of organic matter. Aso in this stage, ammonia in the sewage is converted to nitrate. This is foowed by a second sedimentation stage to remove soids produced by the microbia activity. The treated effuent shoud be cear, free of pathogens and safe to discharge into a river or possiby be reused for irrigation. 6.5 Suage management What is suage? Described in Study Session 4, suage is water that doesn t contain any faeca matter or urine. Typicay, it is wastewater that is produced from washing ourseves, washing cothes and from food preparation. Many peope simpy throw suage onto the ground or into the street. In sma quantities this may be acceptabe, but in densey popuated urban areas proper handing and disposa is required. Some of the disadvantages of improper disposa of suage incude the potentia to contaminate the soi, poute water sources and create favourabe breeding conditions for disease vectors. 17 of 24 Wednesday 30 March 2016

18 6.6 Stormwater management Which vectors do you think might be encouraged by suage coecting on the ground? Mosquitoes are ikey to be attracted, as they use stagnant water as breeding sites. Fies and rats might aso appear, because the suage woud be a source of drinking water for them. Suage can be discharged to sewers or septic tanks in areas where these faciities exist. Where they do not, it is necessary to construct a pit near the househod to dispose of suage propery (Figure 6.16). The pit shoud be fied with grave or sand. A suage pit keeps the wastewater in one pace and encourages it to soak quicky into the ground. Figure 6.16 Pit for disposa of suage 6.6 Stormwater management In Study Session 4 you earned about the poutants ikey to be present in surface water run-off. What were they? Faeca matter, soi, rubber from tyre wear, itter and oi from vehices. Stormwater has a pouting potentia, as we as being a possibe cause of fooding. In towns and cities, stormwater shoud be directed into stormwater drains (Figure 6.17). 18 of 24 Wednesday 30 March 2016

19 6.6 Stormwater management These shoud be kept cear of rubbish. Cimate change means that many areas are experiencing heavy and proonged rainfa, eading to fooding when the stormwater drains are unabe to cope. Figure 6.17 Stormwater drain designed to prevent fooding in Adama. Sustainabe drainage systems (SuDS) are an aternative to arge storm drains. These systems incude a range of techniques designed to reduce the speed of surface run-off, encourage infitration into the ground and reduce the risk of fooding without the use of any mechanica equipment. Buiding ponds or hoding areas in the drainage system so that storm water is temporariy stored and the speed of fow is reduced is one exampe. These storage ponds aso make the water avaiabe for reuse. Another exampe is to encourage infitration by using grave or stones rather than soid concrete at the sides of roads (Figure 6.18). This reduces surface run-off and heps to repenish groundwater. 19 of 24 Wednesday 30 March 2016

20 Summary of Study Session 6 Figure 6.18 ground. Infitration trench with oose stone that encourages water to soak into the Summary of Study Session 6 In Study Session 6, you have earned that: 1 Pit atrines are the most common method of disposing of human wastes in Ethiopia. 2 Faeca sudge accumuates in pit atrines and septic tanks and must be reguary removed and disposed of safey. 3 The sudge can be removed by various means, incuding vacuum truck, Vacutug or by manuay operated pumps such as the Sudge Guper. 4 Sudge can be spread on and as a soi conditioner, but ony after it has been treated to minimise heath risks, for exampe by drying, composting or anaerobic digestion. 20 of 24 Wednesday 30 March 2016

21 Sef-Assessment Questions (SAQs) for Study Session 6 5 Some water-borne toiet systems use septic tanks which hod wastewater for a minimum of 24 hours so that soids sette out. The effuent is discharged into the soi via a soakaway or drainfied. Septic tanks need to be desudged when approximatey one-third fu. 6 The aims of wastewater treatment are to reduce the amount of biodegradabe matter and soids in the effuent, remove toxic materias and eiminate pathogenic microorganisms. 7 Waste stabiisation ponds and reed beds use natura systems to treat wastewater and are suitabe for warm countries. They do not need mechanica equipment and are thus ow in operationa costs but they both require a ot of and. 8 Mechanica-bioogica wastewater treatment systems require ess and than natura systems of wastewater treatment but they require equipment that makes them expensive to insta and operate. 9 Suage shoud be discharged to sewers or septic tanks. If these are not avaiabe, it shoud be discharged into a pit fied with grave or sand. 10 Stormwater can be managed by sustainabe drainage systems. Sef-Assessment Questions (SAQs) for Study Session 6 Now that you have competed this study session, you can assess how we you have achieved its Learning Outcomes by answering these questions. SAQ 6.1 (tests Learning Outcome 6.1) Match the foowing words to their correct definitions. drainfied a network of perforated pipes through which the outfow from a septic tank goes into the ground Sudge Guper a manuay operated pump for desudging pit atrines anaerobic ponds first stage of waste stabiisation pond system treating highy pouted wastewater septic tank a watertight tank instaed underground that coects sewage in areas where there are no sewers facutative ponds ponds that have oxygen at the top and no oxygen at the bottom comminutors an item of equipment that is used to cut up paper and rags in wastewater fowing into a treatment pant Vacutug a sma vacuum truck designed for pit atrine emptying in crowded areas 21 of 24 Wednesday 30 March 2016

22 Sef-Assessment Questions (SAQs) for Study Session 6 soakaway a pit into which the wastewater from a septic tank goes for disposa maturation ponds ponds that are used at the end of a waste stabiisation system to reduce the eve of pathogens in the wastewater retention time the time that is spent in a given environment desudging the process of removing sudge from a pace sustainabe drainage systems drainage systems that encourage infitration of rainwater without specia equipment faeca sudge management combined processes for safey disposing of sudge from pit atrines and septic tanks SAQ 6.2 (tests Learning Outcome 6.2) Outine the main advantages and disadvantages of vacuum trucks, the Vacutug and the Sudge Guper as options for pit emptying. Answer Vacuum trucks are quick and efficient, but because they are arge vehices are not suitabe in some urban areas where roads and passageways are too narrow for them. Vacutugs are smaer and therefore can access paces that trucks cannot reach. In paces with very imited space a Sudge Guper or simiar hand-operated pump may be a better option. These are very simpe toos, but it may be difficut to ensure the safety of the operator and effective containment of sudge throughout the process. SAQ 6.3 (tests Learning Outcome 6.3) (a) (b) List the points where sudge is produced in the different treatment processes described in this study session. State briefy the options for treatment and reuse of the sudge. Answer (a) (b) Sudge is found in pit atrines, septic tanks, anaerobic ponds in waste stabiisation pond systems, and in primary and secondary sedimentation tanks in mechanicabioogica treatment pants. Some options for treatment and reuse are: direct appication onto and as a soi conditioner after two years of storage drying out the sudge composting the sudge with vegetabe matter anaerobicay digesting the sudge to produce biogas. 22 of 24 Wednesday 30 March 2016

23 Sef-Assessment Questions (SAQs) for Study Session 6 SAQ 6.4 (tests Learning Outcome 6.4) The tabe beow has severa statements appicabe to different wastewater treatment systems. Aocate each of them to one of the foowing wastewater treatment options: Septic tank Waste stabiisation ponds Reed beds Mechanica-bioogica treatment. Is expensive due to tanks and equipment invoved Can be used to treat wastewaters containing pesticides Reies on sunight and wind for treatment Need a ot of space Requires skied personne for operation Used for houses with water-fushed toiets in areas without sewers The area around the roots and rhizomes contains aerobic and anaerobic bacteria A vacuum truck is used to suck sudge out of it Consists of many different stages of treatment Partiay treated wastewater goes to a soakaway or drainfied A symbiotic reationship exists between the bacteria and agae in this system Use pants for treatment of sewage Answer Is expensive due to tanks and equipment invoved Can be used to treat wastewaters containing pesticides Reies on sunight and wind for treatment Need a ot of space Requires skied personne for operation Used for houses with water-fushed toiets in areas without sewers The area around the roots and rhizomes contains aerobic and anaerobic bacteria Mechanica-bioogica treatment Reed beds Waste stabiisation ponds Waste stabiisation ponds Mechanica-bioogica treatment Septic tank Reed beds 23 of 24 Wednesday 30 March 2016

24 Sef-Assessment Questions (SAQs) for Study Session 6 A vacuum truck is used to suck sudge out of it Consists of many different stages of treatment Partiay treated wastewater goes to a soakaway or drainfied A symbiotic reationship exists between the bacteria and agae in this system Use pants for treatment of sewage Septic tank Mechanica-bioogica treatment Septic tank Waste stabiisation ponds Reed beds SAQ 6.5 (tests Learning Outcome 6.5) You are waking in an aey in an unsewered part of town when you are neary drenched by washwaters from cothes washing thrown over a fence. What woud you do to advise the househoder about proper disposa of their wastewater? Answer After recovering from my shock, I woud approach the house where the water came from and genty expain to the person who threw the water that it is not advisabe to dispose of such waters (suage) by throwing it away as they did because: an innocent person coud get soaked with dirty water the washwaters on the ground coud become a breeding pace for mosquitoes, and fies and rats coud aso be attracted to it as a water source the washwaters woud make the aeyway wet and unpeasant. I woud suggest that the best way to dispose of the suage woud be to pour it into a hoe that has had grave or sand put into it. SAQ 6.6 (tests Learning Outcome 6.6) Name three ways in which the risk of fooding from stormwater can be reduced. Answer Food risk can be reduced by buiding storm drains that are arge enough to cope with arge voumes of water constructing ponds that wi temporariy hod the stormwater using broken stones and grave rather than soid concrete in roadside ditches and other paces where rainwater buids up so that water can infitrate into the ground. 24 of 24 Wednesday 30 March 2016