Appendix H Wastewater Treatment Plant

Transcription

1 Appendix H Wastewater Treatment Plant

2 New Mauritius Hotels Ltd Les Salines Sewage Treatment Plant 1. INTRODUCTION New Mauritius Hotels Ltd proposes to construct Les Salines Golf & Resorts comprising a hotel of 250 Nos. rooms and 60 Nos. Apartments each of 3 bedrooms. It is proposed that all the wastewater from the hotel will be collected via a sewer reticulation networks and the wastewater treated via an RBC system to tertiary level so that the treated effluent can be reused for irrigation of the landscaped areas within the resort premises. The scope of Works shall be the treatment plant itself and the transfer system for the treated effluent to the irrigation water storage tank but does not include the conveyance system from the hotel to the treatment plant or the irrigation system SELECTION OF TREATMENT PROCESSES Two main types of treatment systems are used in the hotel sector: the activated sludge system-as (including its various variants) and the Rotating Biodiscs Contactor system. A technical comparison of the AS and RBC systems is given in Table below. Table 1.1.1: Technical Comparison between the RBC and Activated Sludge Systems Features Activated Sludge System Rotating Biodiscs Contactor System Sludge Production Excess sludge needs to be removed on a daily basis. Hence drying beds are needed. Moving Parts More mechanical parts operating Bulking Sludge and Foaming Ease of Operation at high speed with more wear Probable. This will cause poor liquid/solid separation with concomitant poor treated effluent quality. Requires qualified personnel with a good understanding of the processes involved. Daily attendance needed. Process Control medium to high Low Minimal sludge produced which are recycled to the septic tank. No drying beds are required. Less mechanical parts operating at slower speed, hence less wear. Has not been observed with domestic sewage. Low level of understanding needed with plant attendance limited to frequent visit. WATERTECH LTD Engineering Water 1

3 Energy High Low Consumption Operating Costs Moderate to high Low Plant surface Area Require more land. Less area needed. From above Table, the RBC appears to be more appropriate than an activated sludge system for sources of wastewater with high seasonal variability as well as the low technical know-how required for the operation of the RBC system among others The Rotating Biodiscs Contactor (RBC) System The RBC system comprises a number of treatment processes in series, namely: Coarse solid and Oil & Grease Removal; Anaerobic digestion; Aerobic digestion; Liquid/solid separation; Chlorination; Filtration; and Phosphate removal by chemical precipitation 2. DESIGN DATA The potable water for the hotel has been estimated and shown in Table 2.1. Table 2.1: Potable Water requirements ITEM NO DESCRIPTION NO UNITS WATER USAGE QUANTITY (m 3 /d) 1 Rooms 430 No 800 l/room Kitchen 2.1 Main 1720 Meals 20 l/meal served Secondary 300 Meals 20 l/meal served 6 3 Personnel 550 No 50 l/person/d Canteen 550 Pax 15 l/meal served Spa and Fitness Centre 5 Swimming Pool Make 6 Up 5000 m l/m2/d Laundry 0 Potable Water Requirements (m3/d) 430 Potable Water Demand (m3/d) 450 WATERTECH LTD Engineering Water 2

4 Treatment of the raw sewage is based on the following design parameters given in Table 2.2 below: Table 2.2: Design Criteria for the STP Description Estimate wastewater flow -ADWF(m 3 /d) Infiltration rate (m 3 /d) 2 50 Total Estimated Flow (m 3 /d) 400 Design flow (m 3 /d) 450 Peak Factor 3 Peak Flow 67.5m 3 /h DESIGN WASTE WATER CHARACTERISTICS COD (mg/l) 750 TKN (mg N/l) 60 Ammonia (mg N/l) 50 Phosphorus (mg P/l) 15 Oil & Grease (mg/l) 150 ph 6.5 Suspended solids (mg/l) DESCRIPTION OF THE VARIOUS PROCESSES Description of the processes and specifications are given below. The processes and specifications take into account the site constraints as described previously. 3.1 Introduction The sewage treatment plant will consist mainly of: An inlet works. A set of anaerobic tanks having a total effective capacity of 675m³. An equalisation tank with effective volume of 130m 3. Parallel sets of RBC units each containing equal number of in series basins of total surface area not less than m 2 ; Two lamellar type clarifiers each with a surface area of not less than 30m 2 or equivalent solid liquid separation system. A chlorine contact tank with an effective volume of not less than 20m³. The contact time within the tank would be at least 15 minutes at peak flow. One filtration system with a filtration rate not exceeding 10m/h. 3.2 Inlet Works Screening of coarse Solids The sewage from the hotel complex is transferred into an inlet channel at the treatment plant. The inlet channel (width: 500mm) contains a screening chamber where a mechanical screen will retain some solids matter from the sewage flow. The mechanical screen shall be designed to cater for a flow of 70m 3 /h. Provision is 1 Assume that 80% of the water requirements reappear as wastewater 2 The total mm.km of wastewater collection system: 1100 and the infiltration rate of 0.045m 3 /d.mm.km is assumed. WATERTECH LTD Engineering Water 3

5 made for four static screens (porcupine screens) which shall be used in the event of non-functioning of the mechanical screen. After the screening chamber, the flow will pass through a small venturi flume which serves to control the level in the screening chamber but will also provide a possible measuring point in the future. The flow then enters a grease removal system Grease Removal System The grease removal system shall be designed for a flow of 70m 3 /h and for a retention time not less than 15minutes. The grease removal system shall be equipped with a fine bubble aerator to float the oil and grease particles. The float shall be skimmed by means of a mechanical skimmer into a launder with sufficient capacity so as to allow the aqueous fraction to drain back to the tank. The oil &grease fraction shall be sent to a storage bin (fibreglass or HDPE) of capacity not less than 0.5m 3. Plastic bags shall be used for collection of the oil & grease and their ultimate disposal with solid wastes. During the operational phase of the plant, the collected float can also be carted away for disposal to a public treatment facility. The nearest facility is at Montagne Jacquot treatment plant that is under the responsibility of the Wastewater Management Authority and authorisation will be required from the latter. From the grease removal system, the effluent is discharged into the anaerobic tanks. 3.3 Anaerobic (septic) tanks The anaerobic tanks shall consist of 4 tanks in parallel, and the total effective capacity shall be 675m 3. These tanks will have common walls and be connected by launders at inlet and outlet. The anaerobic tanks shall be totally closed with only a minimum of four access manholes per compartment. 3.4 Equalisation tank From the anaerobic tanks, the effluent overflows into a flow equalisation tank. The tank is designed to regulate the flow to the RBC Units and shall have an effective capacity of 130m 3. Two submersible pumps (one duty, one standby) will transfer the effluent from the anaerobic tank lifting station to the RBC distribution man hole. A time controller as well as float switches will control the operation of these pumps. Each pump shall have a capacity of 11l/s (40m 3/ h). Each pump shall be equipped with a manometer and other accessories such as nonreturn valve, gate valve, etc. One mixer/aerator will provide the mixing and aeration of the contents of the equalisation tank. The intensity of mixing shall not be less than 40w/m 3 with a residual oxygen content of 2mgO 2/l. 3.5 Rotating Biodiscs Units From the distribution box, the effluent is distributed equally to the sets of RBC units in parallel, each containing in series RBC units and basins. WATERTECH LTD Engineering Water 4

6 The biodiscs shall be made of a material that promotes the formation of a thick slime or biological film. Each biodisc shall have a diameter not exceeding 2.5m. The biodiscs total surface area shall not be less than 15876m 2. The biodiscs unit configuration shall be such that one set of in-series basins can treat 50% of the total daily flow. The speed of rotation of the biodiscs shall not exceed 10 rpm. The shaft supporting the biodiscs shall be made of steel. The effluents from the RBC basins are discharged into a collection/distribution manhole and then to the clarifiers for liquid solid separation. 3.6 Odour Control System for the STP Building Introduction As the sewage from the hotel complex will transit in several lifting/pumping stations before it arrives at the inlet works, fermentation may occur with the probable generation of foul smell when the sewage is discharged at the inlet works. Moreover, as the STP is located within a Built Area, the STP will be housed inside a building. Moreover, fermentation in the anaerobic tanks can also generate and release noxious gases. These gases can be in dissolved and gaseous form and hence the RBC unit can also be a source of fouls smell. To prevent odour emanation from the inlet works, anaerobic tanks and RBC units, these units will be housed inside a building enclosure and all gases from the building shall be scrubbed prior to their release to the atmosphere. The odour control system shall be a set of biological filters and includes also the following: Centrifugal fan (one duty, one standby), Interconnecting duct works, and Other appurtenances; so that the system is completely operational. The odour scrubber system shall remove all foul gases including but not limited to hydrogen sulphide, mercaptans, and volatile organic compounds (VOC), etc. The air turnover rate inside the building enclosure shall be not less than 4 volumes per hour. Two centrifugal fans (two duty, one standby) shall feed the vented air to the bio filters via pressure ducting/pipes. The vented air shall enter the bio filter via an exhaust pipe that connects to a perforated pipe system at the bottom of the bio filter. The distributed air flows upwards into the media and the odorous compounds are adsorbed on the media and ultimately the micro organisms present in the media degrade the odorous organic compounds. The odour free air is then released to the atmosphere via the top of the bio filters. Provision is made for at least three biofilters. WATERTECH LTD Engineering Water 5

7 3.6.2 Centrifugal (Extractor) Fans Two centrifugal fans (two duty, one standby), each shall have a capacity of at least three turnovers per hour of the volume of the building. The material of construction for the fan and its parts shall be resistant to the corrosive nature of the surrounding environment. Material of construction shall be stainless steel, fibreglass or any approved corrosion resistant material. Each extractor fan shall be installed in a duct that leads to the biofliters. The building enclosure shall also have adjustable louvres positioned in such a way so as to provide a cross draught across the enclosure Biological Filter The contaminated air shall be driven into a matrix comprising one or more layers of biologically active materials. The contaminants in the air diffuse through the bio film, where aerobic degradation in the presence of micro organisms. Organic compounds are converted into carbon dioxide and water whereas hydrogen sulphide (an inorganic compound) is oxidised to inorganic acid. 3.7 Clarifiers Provision is made for two lamellar type clarifiers each with a surface area not less than 30m 2. Each clarifier shall be provided with scum board to minimise scum discharge and V-notch weirs to evenly distribute the overflow over the entire periphery. A scum box shall be used to collect floatable matter on the surface of the clarifier; the scum box shall discharge into a bin Effluent will pass over the V-notch weirs into a launder, from where it shall be discharged directly to the chlorine contact tank. Sludge collected at the bottom of the clarifiers will be pumped back to the anaerobic tanks. Each pump (one duty, one standby) with a transfer capacity of 2.78l/s (10m 3 /h) will transfer the settled sludge. The clarifier is also designed to allow settling of chemically precipitated phosphate ions. 3.8 Nutrient Removal The RBC system should allow the denitrification process to take place within its processes. Furthermore, phosphate will be chemically precipitated as indicated previously. The maximum permissible limits for these two nutrients are given in Section Chemical Precipitation of Phosphate Phosphorus removal by the micro organisms will be due to uptake for metabolism only. Only about 4.5mgP/l will be removed through this pathway. The remaining phosphorus in the wastewater will be removed by coagulation with aluminium phosphate/poly aluminium chloride. Aluminium sulphate/ poly aluminium chloride (ALUM/PAC) will be dosed in the effluent boxes (distribution man hole for the clarifiers and RBC units) where rapid mixing takes place. WATERTECH LTD Engineering Water 6

8 Provision shall be made for two injection points, one for each effluent box. Each injection line shall be fitted with a gate valve to enable control of dosage. The phosphorus precipitates will then settle in the clarifier and return to the anaerobic tanks. This precipitate will be enmeshed in the sludge mass. The dosing tank capacity will be at least 1000 litres and will be housed in the building. There shall be two dosing pump sets (one duty, one standby) to dose the coagulant into the effluent boxes. Each dosing set will consist of a polyethylene mixing tank, electric mixer, and variable dosing pump, all suitable for aluminium sulphate/ poly aluminium Chloride (ALUM/PAC). The dosing pump set (one duty, one standby) shall have a capacity of delivery of 0 to 30litres/hr. The mixing tank shall have a drain, removable porthole for preparing the solution. The dosing pump shall be fixed on the wall of the control room De nitrification The nitrate generated during the aerobic stage of the process (RBC units) will need to be reduced to nitrogen gas so that the maximum permissible limit for nitrate concentration as per local standards is observed. De nitrification will be achieved by recycling the underflow of the clarifiers to the anaerobic/anoxic zones of the anaerobic tanks. 3.9 Chlorination and Chlorine Contact Tank The chlorine contact tank will consist of two sections: one section for chlorination and the other section will be a buffer storage; the total effective volume shall be approximately 25m³. The tank will be provided with baffle walls to minimise short- circuiting and to ensure sufficient contact time with the chlorine. The buffer storage provided in the tank will be sufficient to allow effluent from this tank to be passed directly through a pressure sand filter system. Two hypochlorite-dosing sets shall be provided (one duty, one standby) to dose chlorine into the effluent in order to maintain a free chlorine residual of between 0.2 and 0.4 mg/l. Each dosing set will consist of a polyethylene mixing tank, electric mixer, variable dosing pump, all suitable for making up a solution of sodium hypochlorite and dosing into the clarifier overflow. The mixing tank shall have a drain, removable porthole for preparing the solution. The capacity of the tank shall not be less than 1000 l. The chlorine contact tank shall be equipped with a walkway and handrails and shall be covered by expanded metal Effluent Filtration Plant A pressure sand filtration system shall be provided to filter the effluent prior to being sent to the irrigation tank (tanks of the existing STP). WATERTECH LTD Engineering Water 7

9 The effluent will be drawn directly from the chlorine contact tank. The pump system shall be controlled by level controls located within the tank. The feed pump shall operate only when the water depth within the chlorine tank is greater than a set minimum water level in the additional storage capacity of the contact tank. A bank of filters working in parallel shall be provided. Back washing shall be automatic and filtered water from the adjacent filter being used for back wash purpose. The back-wash water shall be discharged into the anaerobic tanks. One duty and one standby pump shall be provided. Each pump shall be fitted with a manometer in addition to other accessories such as non-return valve, gate valve, etc. The filters shall be manufactured in a corrosion resistant material and shall be sized to cater for a normal flow of 6.95 l/s. The filtration rate shall not be more than 10 m/h. The filter media specification is dual media quartz/anthracite media with a bed depth not less than 800mm. Inlet and outlet pressure gauges shall be fitted to the filters and isolated by suitable gauge cocks Excess Sludge Treatment and Disposal The treatment system will generate excess sludge during the aerobic degradation phase (RBC systems). The excess sludge will be sent to the anaerobic tanks where it will be digested anaerobically. Over a period of time, there will be an accumulation of sludge mass (non - biodegradable fraction) in the anaerobic tanks and this needs to be removed from the tanks. Due to the location of the STP within a built area, tankers will be commissioned for de-sludging. It is estimated that the frequency of de sludging shall be once every three months (assuming a hotel occupancy rate of 90%). Provision will be made to facilitate the desludging process. Pipe works and accessories (valves etc.) will be installed such that connection with a desludging tanker and the desludging process is rapid. WATERTECH LTD Engineering Water 8

10 4. SCOPE OF SUPPLY FOR ELECTRICAL & MECHANICAL EQUIPMENT 4.1 Mechanical Screen As described 4.2 Grease Removal System As described 4.3 Odour Scrubber System As described 4.4 Pumps Equalisation tank: Submersible Pumps Number: 2 Capacity: 11l/s Head : 3m Accessories: Guide rails, discharge outlet, float switches, and non-return valve Sludge Recycling Pumps (for lamellar type clarifier) Number: 3 (two duty, one standby) Capacity: 2.78l/s Head : 4m Accessories: Guide rails, discharge outlet, float switches, and non-return valve Filtration Pumps Number: 2 (one duty, one standby) Capacity: 6.95 l/s Head : 14 m Accessories: Suction pipe, discharge outlet, float switches, and non-return valve De Sludge Pump Number: 1 Capacity: 5.56l/s Head : 4m Accessories: Guide rails, discharge outlet, and non-return valve. 4.5 RBC Units Number: (to be specified by Tenderer) No of discs per unit: (to be specified by Tenderer) Minimum Diameter of one disc: (to be specified by Tenderer) Material of disc: Injection moulded Polyurethane Surface area of one unit: (to be specified by Tenderer) Speed of rotation: (to be specified by Tenderer) Motor Power of one Unit: (to be specified by Tenderer) Total Motor Power: (to be specified by Tenderer) 4.6 Lamellar Clarifiers Number: 2 Type: Static Accessories: scum board, peripheral weir, static scum box. 4.7 Chlorine Dosing System Dosing Pump: WATERTECH LTD Engineering Water 9

11 Number: 2 Capacity: 0-30 l/hr Type: Positive Displacement Dosing Tank Number: 1 Capacity: not less than 1000 litres Accessories: electric mixer, outlet pipes etc. 4.8 Aluminium sulphate/poly aluminium chloride Dosing System Dosing Pump: Number: 1 Capacity: 0-30 l/hr Type: Positive Displacement Dosing Tank Number: 1 Capacity: not less than 1000 litres Accessories: electric mixer, outlet pipes etc. 4.9 Filtration System Filter : Glass reinforced Polyester Diameter : to be specified by the Tenderer Height : to be specified by the Tenderer Bed Depth : to be specified by the Tenderer Media : silica sand/anthracite (50%/ 50%) Filter media granulometry : 0.95 to 2.0 mm Bank quantity : to be specified by Tenderer Filter per bank : to be specified by Tenderer Filtration rate : not more than 10 m/h (av.) 4.10 Porcupine Screens Four (4) 4.11 Mixer/Aerator for Equalisation tank (1No) 4.12 MCC There shall be three control panels: MCC1 This will be the main control panel and will provide power supply to all the other control panels MCC2 This control panel will be located in the building enclosure and will provide control for the following equipment: Mechanical screen (1No) Aerator of the grease removal system (1No) Scraper system of the grease removal system (1No) Pumps for the equalisation tank (2Nos) Mixer/Aerator for equalisation tank Centrifugal extractor fan of the odour scrubber (2Nos) WATERTECH LTD Engineering Water 10

12 MCC3 This control panel will be located in the control section and will provide control for the following equipment: Filtration system Dosing system for hypochlorite (1 dosing pump and one electric mixer) Dosing system for Aluminium sulphate/pac (1 dosing pump and one electric mixer) For each M&E equipment in the respective control panel, the following accessories are required: A running meter, A time controller. Other accessories to be included in each control panel are given in the M&E specifications (see section 2). 5. TREATED EFFLUENT QUALITY For the Rotating Biodisc Contactors (RBC) system proposed in the previous sections, the anticipated effluent quality after chlorination and filtration should satisfy at least effluent standards for irrigation in force in Mauritius as per section Effluent Standards in Mauritius Standards for the discharge of effluents as irrigation have been promulgated under the Environment Protection Act of The main parameters are reproduced in Table Table 5.1.1: Effluent Quality Standards- Pertinent Parameters* 1 Parameters 4 Unit Concentration* 2 Temperature o C 35 ph Suspended Solids mg/l 45 BOD o C mg/l 40 COD mg/l 120 Free chlorine (as Cl 2) mg/l Phosphorus (as PO 4 ) mg/l 5 Nitrate (as N) mg/l 20 TDS mg/l 2000 Oil & Grease mg/l 10 Faecal coliforms 3 MPN per 100ml 200 Intestinal nematodes No of eggs per 100ml <1 Note: 1: Parameters referring to metals, phenol and pesticides have been omitted as they are not pertinent to this type of wastewater. 2: Refers to maximum permissible concentration. 3: For public lawns such as hotel lawns with which the public may have direct contact. 4: A 95% compliance limit will be accepted on the series of samples taken in a year. WATERTECH LTD Engineering Water 11

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