OPTIMIZING PERFORMANCE OF WATER TREATMENT PLANTS by K.B. Wadhavane Executive Engineer, WTP, MCGM

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1 OPTIMIZING PERFORMANCE OF WATER TREATMENT PLANTS by K.B. Wadhavane Executive Engineer, WTP, MCGM

2 INTRODUCTION Mumbai is metro city and financial capital of India having population over 12.5 million. Total water supply to city Mumbai is 3753 Mld. In all there are seven sources of water Vihar, Tulsi, Tansa, Vaitarna, Upper Vaitarna, Bhatsa and most latest Middle Vaitarna lake and 5 Water Treatment Plants to cater need of Mumbai City. At Bhandup Complex two WTPs of 1910 Mld and 900 Mld capacity are installed which is largest in Asia. One each of WTP of capacity 18 Mld, 90 Mld and 1350 Mld are at Tulsi, Vihar and Pise Panjrapur respectively. The water supply network is developed over 130 years and 7th largest in the world. The focus of this paper is optimizing performance of Water Treatment Plants (WTPs) by highlighting challenges faced and probable solutions.

3 INTAKE WELL To fetch water with good amount of dissolved oxygen, upper side gates shall be opened. Depth of the gate shall be such that it restricts entry of algae to water conveyance system. Opening lower side gates will result into entry of dead zone water which will lead to colour issues.

4 PRECHLORINATION Chlorine demand:chlorine demand test shall be carried out to ensure residual chlorine of 0.2 to 0.4 mg/ ltr. when water enters to water treatment plant. The chlorine contact time shall be more than 30 min. for proper effect of chlorination. ph plays vital role in effectiveness of chlorination. If ph equal to or less than 7, presence of HOCl is more ensuring better disinfection and destroying of algae. Alkaline ph results into more Ocl ions and chlorination becomes less effective. HOCl is almost 100 times more effective than Ocl.

5 AERATION Normally community water treatment draws water from river and lakes. As such many times sufficient dissolved oxygen is available and aeration is not required. Unnecessary provision of aeration by cascade aerators or diffused aerators results into head loss and if treatment involves raw water pumping is then energy loss occurs for life time of the water treatment plant. The necessity shall be decided on case to case basis. Air induction method can be utilized by use of energy from flowing stream which will save on electrical power.

6 PAC DOSING TECHNIQUE Jar testing : Jar testing shall be carried out on regular basis to decide on correct dose of coagulant. Jar testing shall also be carried out whenever there is a change in raw water quality. Powder PAC dosing : Preparing and ensuring proper dose of Poly aluminium chloride ( PAC ) by use of powder PAC requires great amount of expertise. The solution shall be prepared taking in consideration the clear tank volume and by use of constant dosing pumps. In general PAC solution shall be prepared by use of mechanical mixer. The solution prepared shall be utilized within 24 hours. Liquid PAC dosing : In most of the WTPs of MCGM, Poly aluminium chloride (PAC) of medium basicity is used as a coagulant. It shall be diluted to increase physical contact and for effective mixing. The dilution arrangement shall be very near to dosing point. In fair season dilute solution and in monsoon concentrated solution shall be prepared. Presently for Pise Panjrapur IIIA WTP and new 900 Mld plant dilution of PAC is carried out at the location very close to dosing point. Dilution in fair season - Solution of 5% concentration of PAC is prepared and dosed. Dilution in monsoon season - Solution of 10 to 12% concentration of PAC is prepared and dosed.

7 OPTIMISING PAC DOSING It is essential to monitor correct PAC dose as per Jar test and experience. It is necessary to cut on cost of chemical and more importantly excess dosing causes residual aluminium content to increase above 0.03 mg/l specified as acceptable limit in Drinking Water Standard BIS This limit is hard to achieve in WTPs. Permissible limit in absence of alternate source is 0.2 mg/l. When PAC is dosed the negatively charged colloidal matter get attached to the positively charged Al ions forming flocs which agglomerate and finally settle down. If the dose is increased beyond optimum dose then instead of attraction there will be repulsion and influent will get disturbed instead of settling. Therefore optimum PAC dose has to be ensured all the time. Jar test shall be resorted to whenever there is change in raw water quality. The dose alteration shall be done in response to flow, temperature and detention time of settlers.

8 EFFICIENT OPERATION OF SETTLERS Ensure uniform distribution of raw water to individual settler. In general coagulant dosing shall be followed by flash mixing. The agitator shall rotate at around 96 rpm to ensure proper mixing of the PAC with incoming raw water. In flocculation zone agitator speed recommended is 6 rpm. However to respond to varying conditions the agitator shall be controlled by Variable Frequency Drive (VFD) to control the speed so that flocs formed are not ruptured. More detention time results in good settling. Periodical desludging will ensure proper health of the settler. Sludge accumulation at the bottom of the settler will result into anaerobic conditions resulting into formation of carbon dioxide, hydrogen sulphide and imparting foul smell to the water. Moreover it will lead to tripping of scrapper arms. This condition also triggers floc eruption due to temperature effect. In case of settlers working on principle of recirculation of sludge, the % of sludge recirculation shall be decided in relation to incoming water turbidity. The sludge recirculation can be upto 25% for higher turbidities. In case of pulsatube type of settlers proper formation and retention of sludge blanket is a key in proper performance of settler. The speed of sludge blanket formation depends on the characteristics of water & on the coagulant doses needed to perform clarification. The sludge blanket may form in three days in turbid raw water & large quantities of coagulants. Conversely the sludge blanket may take more than 15 days with low turbid water.

9 EFFICIENT FILTER OPERATION For proper functioning of filter bed the filter media of required specifications shall be ensured and whenever loss in filter media is more than 5%, it shall be topped up. Final water quality of each filter bed shall be monitored and corrective action shall be taken. Ristrictor valve position shall be adjusted such that desired turbidity levels are achieved. In case of rapid sand filters entire depth of the filter bed is responsible for removal of suspended particles. Even then top 6 inches layer of filter media helps the most in filtration process. Very often time based backwashing of filters results in breaking of good filtration process when it is at it's peak. Ideally filter backwashing shall be carried out based on head loss. Filter shall be automated to carry out backwashing based on head loss. Turbidity spikes observed during filter ripening after filter back wash are best managed by filter back washing based on head loss. Filter Automation- Advantages of Filter automation are 1. It saves manpower. 2. Back washing based on head loss reduces water loss by increasing filter run. 3. Better water quality is ensured as no unnecessary backwashing. 4. Saves power as no unnecessary running of blowers. Entire new 900 Mld plant is automated including filter automation based on head loss. Head loss criteria for backwashing is fixed as 1.2 MWC.

10 WATER LOSS MANAGEMENT BY FILTER BACKWASH & DESLUDGE WATER RECYCLING In general in any water treatment plants water losses on account of desludging of settlers and backwashing of filters can be upto 4%. Recycling of waste water makes WTP zero liquid discharge and results in increase in community water supply. i. Filter Backwash Water Recycling Plant for 1910 Mld WTP: In order to make WTPs zero liquid discharge, it is essential to recycle filter backwash water and supernatant water from Sludge Recovery Area. It is one of the smart city objectives. Accordingly under Mumbai IV project, 40 Mld plant to recycle filter backwash water of old 1910 Mld WTP plant is constructed along with new 900 Mld plant. New 900 Mld plant has its own filter backwash recycling plant of 20 Mld capacity. The system consists of collecting filter backwash water by use of sand trap, balancing tank, mixers and sump pumps to recycle water back to inlet bay.

11 WATER LOSS MANAGEMENT BY FILTER BACKWASH & DESLUDGE WATER RECYCLING ii. Common Sludge Recovery Area (SRA) Plant for old 1910 Mld WTP and 900 New WTP (Plant Capacity = 60 Mld) :The plant consists of collecting desludge water having sludge consistency (solid content) around 0.5% from pulsatube settlers of new 900 Mld plant and pretreators of old 1910 Mld plant. The sludge water is collected in common sludge balancing tank and pumped to flocculator where in polyelectrolyte dose of 3 to 5 ppm is added for thickening. The influent is then fed into 4 nos. thickeners. Sludge of 4% consistency from thickeners is taken to sludge balancing tank from where it pumped to 10 nos. of centrifuges for 2 ndstage process by screw pumps. Centrifuge converts 4% sludge of 1 ststage into 20% sludge consistency. Polyelectrolytes dose for centrifuge is between 95 ppm to 100 ppm. The sludge cake formed is then taken for disposal by trolleys. The supernatant from thickener and centrate from centrifuge is taken to common balancing from where it is pumped back to inlet bay of 1910 Mld plant for reuse.

12 WATER LOSS MANAGEMENT BY FILTER BACKWASH & DESLUDGE WATER RECYCLING iii. State of Art Technology, Water Reclamation Plant for 1350 Mld Panjrapur WTP: The salient features of project are : 1. The plant is designed to treat backwash water from filters as well as desludge water from settlers of all three stages. 2. The backwash water of filters will be pretreated by ozonization process as it will contain pathogens entrapped in filter media. This is as per guidelines of US EPA. 3. Filter backwash and clarifier sludge treated in same plant but still separately. 4. Decanter type centrifuges are utilised for continuous solid liquid separation.

13 UV TREATMENT Additional Disinfection by Ultra Violet (UV) Radiation : Implementation of UV Radiation after filtration and before post chlorination is under consideration for added disinfection. Such improvement in existing WTPs is carried out in many advanced countries. The advantage would be, it will take care of viruses cryptosporedium and giardia which are not killed by chlorine also chlorine consumption will reduce.

14 POST CHLORINATION Main objective of post chlorination is to maintain residual chlorine in the distribution network so that it will take care of any possible contamination and citizen will get wholesome water. As per the BIS the residual chlorine at customer end shall be 0.2 mg/ltr. At Bhandup Complex WTP residual chlorine of 1 to 1.5 mg/lit. is maintained considering the length of the distribution network supplying water to city of Mumbai. Normally chlorine solution lines are placed at strategic locations to ensure uniform chlorination to large volume of water. The operator shall ensure equal amount of chlorine dose through solution lines and see that each of point or alternative point gets chlorination. More dosing shall not be done through single point as it will result in non uniform distribution. The baffles provided inside the chlorine contact tank shall be repaired from time to time to ensure minimum 30 minutes chlorine contact time.

15 EFFICIENT OPERATION OF PUMPING STATION 1. Mitigation measure for Cavitation effect: Replace phosphor bronze impellers by stainless steel impellers having comparatively tough material to sustain cavitation problem. Whenever pumping plant utilisation is less, try to put alternate pumps in service. 2. Keep Bypass valves of suction valve, non return valve & delivery valves closed to avoid losses when pump is out of service. 3. Ensure dripping of water from stuffing boxes - Proper lubrication - Prevents air entry into the pump. 4. Avoid dry running of pump. 5. Starting and stopping of pump with delivery valve closed.

16 EFFICIENT OPERATION OF PUMPING STATION 6. For better efficiency, pump need to be operated at duty point. 7. Avoid number of hot starts as it strains motor insulation. The switching surges reduces motor life 8. Avoid simultaneous switching of pump sets - Starting current ( I start) = 6 x I Normal. - Switching surges will get added - Hydraulically, it will not allow system to stabilize. 9. Avoid delivery valve throttling. Power loss due to friction. 10. For regular operation Use most efficient pumps out of the available lot of the pumps installed in parallel. 11. In case of vertical turbine pumps ensure submergence as specified. Ensure uninterrupted flow of cooling water for thrust bearings.

17 EFFICIENT OPERATION OF PUMPING STATION 12. Managing effect of Power frequency on pump performance:n ( RPM) = 120 f ( Frequency) p ( No. of poles) 1) N α f 3) Q ( Discharge) α N 2) H ( Head) α N² 4) P( Power) α N³ If the frequency remains consistently above or below 50 Hz., the Impeller can be trimmed or Impeller dia. ( D ) can be increased by adding rings for better efficiency. 1) Q α D² 2) H α D² 3) P α D Managing effect of water hammer : In case of sudden power failure, pump tripping occurs resulting into water hammer. The water hammer effect can be best managed by quick closing of non return valves.

18 ENERGY EFFICIENCY IN WATER TREATMENT PLANTS Opportunities for Improving Energy Efficiency in Pumping Station : i. Pump Coating: One pump out of eleven 246 Mld horizontal split casing pump of size 36" 36" is coated with anti friction coating on pilot basis. On measurement of efficiency before and after coating, it is concluded that efficiency is increased by 9.7% and power saving achieved is 7%. On reinstalling pump it is observed that there was reduction in current from 102 Amps to 95 Amps ( at 3.3Kv). Considering the advantage work for balance pumps is in progress. ii. Refurbishment of 620 KW motor for efficiency improvement : The key element in efficiency improvement was that the initial coils of motor had small cross section and thicker B class insulation. New coils placed in same slot had bigger cross section and less thick F class insulation. Therefore overall resistance of motor reduced and efficiency increased. Efficiency of 620 Kw rating motors increased by about 3% after refurbishment. iii. Power factor improvement by installing APFC Panel: All the major pump-motor units has fixed capacitor bank for power factor improvement in parallel. However to account for varying load Automatic Power Factor Control (APFC) panel is installed in addition to fixed capacitor bank. The power factor is improved beyond This has resulted in additional incentive 2% in tariff and MCGM is benefited to the tune of around 7 lakhs per month.

19 ENERGY EFFICIENCY IN WATER TREATMENT PLANTS Solar Power Project for Water Treatment Plant : 110 Kv Sub Station caters for power requirement of old 1910 Mld, new 900 Mld plant and SRA plant. 110 Kv is stepped down to 3.3 Kv for pumping station and 3.3 Kv is stepped down to 415 V for auxiliary supply. Average monthly power requirement of all plants is around 6 MW and monthly electricity bill is about 3.5 crores. For any plant, energy conservation, Green Initiative and self sustainability shall be top agenda of management. Accordingly, MCGM has taken up an ambitious solar power project for Bhandup Complex WTP. Feasibility study and Draft Project Report (DPR) to have solar power project was carried out under guidance of IIT Bombay. Generation potential as per DPR at Bhandup Complex is 12.5 MWp. Areas to be utilised for installation of solar panels are identified as Administrative & MCC building tops, Pumping station, Filter Stream+ MBR+CCT (old and new). DPR envisages total saving of around 30%. In first phase, 2.5 MW solar power project work is taken up on Pilot basis and the work is in progress.

20 CONCLUSION Each treatment process shall be monitored as per standard operating procedure. Experienced and trained Engineer shall be designated as Process Engineer for WTP to optimize water treatment process. The objective shall be to ensure final water quality is consistently compliant to regulatory norms, less consumption of water treatment chemicals, zero waste liquid discharge from water treatment plant. Frequent operations shall be avoided as it may lead to more disturbance. PAC shall be diluted and dosed very near to dosing point for better mixing. PAC/coagulant dosing shall be monitored closely to avoid excess dose so as to keep residual Aluminium levels in final water below specified limits. Automation of filter backwashing based on head loss criteria is most useful as it increases filter run and ensures better water quality. Look for opportunities for energy conservation in pumping station, as normally it consumes over 90% of total power of WTP plant and available options are coating to impellers, refurbishment of big size motors and APFC panel. Normally WTPs have open spaces over buildings, Chlorine Contact Tank, Master Balancing Reservoirs etc., this can converted into green opportunity by installing solar power plant. Ideally, filter backwash recycling plant shall be equipped with disinfection followed by sludge removal.

21 THANK YOU