Report on investigation of manganese in Asaka Water Purification Plant in FY2006

Transcription

1 Report on investigation of manganese in Asaka Water Purification Plant in FY2006 Bureau of Waterworks, Tokyo Metropolitan Government Introduction: The advanced water purification facility started operation in Asaka Water Purification Plant in November After operation brownish-red bubbles appeared on the water surface in post-stage filter basin. The bubbles coming up on the water surface contaminated with manganese hardly affected the quality of the filtrate water. However, the bubbles appearing in the final-stage filter basin of the advanced purification system were likely to give a sense of discomfort to the visitors. We, therefore, investigated the source of manganese and made a thorough study on the countermeasures thereof. The report sums up the investigations made on the supply of manganese from raw water, effect of the returned water from washwater drainage system and wastewater treatment system, manganese concentration in the concerned soil, etc. The report, therefore, clarifies the balance (incoming and ongoing) and circulation of manganese in the water purification plant and describes the countermeasures thereof. Fig. 1 shows the flow chart of the purification treatment in Asaka Water Purification Plant, while Photograph-1 shows the brownish-red bubbles floating in the post-stage filter basin. Raw water From Ara River Receiving well (1 million m 3 ) Returned water (7,000 m 3 ) Sulfuric acid, Pre-chlorination, PAC Intermediate chlorination, Post-PAC Post-chlorination, Caustic soda Normal treatment system (0.2 million m 3 ) Settling basin (8 basins) Filter basin (24 basins) Purified water Washwater drainage (54,000 m 3 ) Wastewater treatment system Advanced treatment system (0.8 million m 3 ) Sulfuric acid, PAC Settling basin (16 basins) Pre-stage filter basin(48 basins) BAC basin (40 basins) Post-stage filter basin(40 basins) Purified water Ozone Waste water treatment plant Sludge 60 t (Moisture content: 60%) Intermediate chlorination, Post-PAC Post-chlorination, Caustic soda Fig.1: Water purification flow in Asaka Water Purification Plant and one day treating volume (1 million m 3 )

2 Photograph-1: Brownish-red bubbles in the post-stage filter basin 1. Examinations and results 1-1 Manganese concentration and form in raw water: Among the four big water purification plants of Tokyo, manganese concentration in raw water in Asaka Water Purification Plant is unusually high as shown in Fig. 2. Since approximately one million m 3 of raw water is treated everyday in Asaka Water Purification Plant, about 90 ~ 100 kg of manganese flows into the water purification plant by calculation. The manganese in raw water is in two forms as shown in Fig. 3: dissolved manganese (soluble type) and suspended manganese (insoluble type). The dissolved manganese gets oxidized by oxidizing agents such as chlorine, ozone, etc. before turning into suspended manganese, which elutes as dissolved manganese in an oxygen-free environment (anaerobic state). Total manganese concentration (μg/l) Asaka Higashi- Kanamachi Murayama Misato Fig.2: Manganese concentration levels in raw water compared Fig. 4 shows the form-wise manganese breakdown in raw water in Asaka Water Purification Plant in The dissolved manganese and suspended manganese breakdown does show monthly variation, but on average it is 40% suspended manganese and 60% dissolved manganese. In terms of daily charging, out of the total 90 kg manganese, 40 kg of suspended manganese and 50 kg of dissolved manganese flow into the water purification plant.

3 Manganese concentration (μg/l) Dissolved manganese Suspended manganese % % 80% 53% 47% 91% 58% % 37% 75% 64% 49% 60% 57% 47% 53% 60% 63% 42% 43% 51% 25% 21% 20% 9% Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Ave Fig. 4: Manganese breakdown in raw water in Asaka Water Purification Plant (FY2005) % 1-2 Examination of manganese in washwater drainage system: The washwater drainage in the filter basin, etc. flows through the washwater drainage basin before returning to the receiving basin for re-use. The filter basins in Asaka Water Purification Plant amount to 152 basins in total: 24 basins in normal treatment (purification) system, 48 basins in advanced purification system (pre-stage filter), 40 BAC basins and 40 post-stage filter basins. (See Fig. 1) Since the effect of washwater drainage on the raw water quality is extremely large, we examined the manganese content in the washwater drainage, with the results of the examination shown in Fig. 5, where the y-axis indicates manganese concentration, with the rates of dissolved manganese and suspended manganese given in %. The total daily volume of returned water from each basin is also indicated. In view of the manganese content in raw water, usually standing at 0.1 mg/l, we can see how outstanding the manganese content is in washwater drainage. The manganese content discharged from the pre-stage filter basin (including normal purification system) and the post-stage filter basin is almost same at 1 mg/l, whereas manganese content from the BAC basin is more than 10 times high. As for the manganese form, it is mostly suspended manganese (with the dissolved manganese ratio being 0.3 ~ 2.8%). This is considered attributed to the addition of oxidizing agents: ozone in the inflow water in the BAC basin, and chlorine in the inflow water in the post-stage filter basin, while in the pre-stage filter basin the suspended manganese in the raw water that could not be treated in the settling tank has been removed.

4 14 12 Dissolved manganese Suspended manganese % % % 97.2% 2.8% 99.7% Pre-stage filter basin BAC basin Post-stage filter basin 22,800m 3 24,000m 3 7,000m3 Fig. 5: Manganese concentration in washwater drainage 1-3 Examination of manganese in wastewater Settling basin treatment system: The sludge discharged from the settling tank flows into the sludge basin in the water purification plant before being divided into the sludge flowing into the regulating tank and the sludge flowing into the concentration tank. Fig. 6 1 Overflow 2 Overflow Sludge basin Regulating tank Concentration tank shows the process in the wastewater treatment 3 Dehydrated filtrate plant and the returned water. During these Filter press dehydrator 4 Filter cloth w ashw ater processes the sludge gets divided into solid and Returned w ater Sludge liquid, turning into sludge and returned water. The supernatant water overflowed from the regulating Fig. 6: Processes in w astew ater treatment plant and returned w ater tank and the concentration tank, the dehydrated filtrate from the filter press dehydrator and the washwater from the filter cloth turn into the returned water from the wastewater treatment system. Arakaw a River Junction well Receiving well Junction square Wastewater treatment plant Inside the purification plant Overflow from regulating tankplant Overflow from concentration tank Washw ater drainage from filter cloth, etc. River w ater Returned w ater Fig. 7: All returned w ater routes Junction square Fig. 7 shows the routes taken by the entire returned water to reach the receiving well. The returned water from the wastewater treatment plant is collected in the junction square, flows into the junction well before immediately flowing into the river. Later, the supernatant water overflowed from the regulating tank and the concentration tank in the plant and the washwater drainage from the washwater drainage system flow together into the receiving well. Wastewater treatment plant

5 The level of manganese content in the returned water from the wastewater treatment system is shown in Fig. 8, with the daily water discharge rate from each discharging source also indicated. The highest concentration level is found in the dehydrated filtrate followed by the concentration tank and regulating tank. Fig. 9 shows the manganese form against the total loading dose of returned water in receiving well, indicating dissolved manganese covering a large portion of 74%. Fig. 10 shows the rate of loading dose per discharging source, with the loading dose being highest in dehydrated filter 3 followed by concentration tank 2 and concentration tank 7. In consideration of the returned water in 1 ~ 4 flowing into the junction square 5, evidently the loading dose from the junction square amounts to 80%. Manganese concentration (mg/l) % 60% 82% 18% 40% 2 Concentration tank 2,260m 3 1 Regulating tank 1,100m % % % 45% 9% 55% 3 Dehydrated 4 Filter cloth 5 Junction filtrate 1,140m 3 washing 680m 3 square 5,580m 3 Dissolved manganese 溶存ンガン Suspended 懸濁ンガン manganese Fig. 8: Source-wise manganese concentration in wastewater treatment plant % 98% 21% 6 Regulating tank 550m 3 0% 20% 40% 60% 80% 100% 2% 7 Concentration tank 1,130m 3 0% 20% 40% 60% 80% 100% 74% 26% Dissolved manganese Suspended manganese Fig. 9 Breakdown of manganese in total loading dose 10% 26% 41% 3% 3% 6% 13% 1 Regulating tank 10% 2 Concentration tank 26% 3 Dehydrated filtrate 41% 4 Filter cloth washing 3% 6 Regulating tank 6% 7 concentration tank 13% Fig. 10: Rate of loading dose per discharging source 1-4 Manganese variation with line in receiving well (predicted) The returned water flow from washwater drainage system and wastewater treatment system is expected to inflict large effect on the water quality in the receiving well. The variation in manganese concentration in the receiving well for one day was predicted, taking account of the operating state of each facility (Table-1). Fig. 11 shows the predicted variation of total manganese in the receiving well with time, and Fig. 12 shows the predicted variation of dissolved manganese with time. The predicted values have been calculated by taking consideration of the manganese concentration levels in Fig. 5 and Fig. 8, and the water volume and operating state in Table-1. Pattern A indicates the day when the water purification plant treats approximately one million m 3 of water per day, the waste water treatment system is in operation, and the washing in the filter basin is carried out using all basins including pre-stage filter basin, BAC basin and post-stage filter basin. Pattern B differs from pattern A only in one point: there is no washing using BAC basin, while pattern C

6 differs from pattern B only in one point: there is no washing using pre-stage filter basin (Table-1). The total manganese concentration in Fig. 11 shows an excessively high level in pattern A, while patterns B and C indicate similar figures, obviously indicating the great effect of BAC basin washing, and the small effect of pre-stage and post-stage filter basin washing. The concentration of dissolved manganese in Fig. 12 is found to be of the same level for all patterns. Since the washwater drainage in Fig. 5 hardly contains any dissolved manganese, it is evident that the rise of dissolved manganese is attributed to the returned water from washwater treatment system. In this way, the change in manganese concentration is seen in the receiving well depending on the time zone, clearly indicating that the cause (of charge) lies with the operating time of the washwater drainage system and wastewater treatment system. Total manganese concentration (mg/l) Table-1: Operation status of each facility and pattern Operation status Junction well (raw water transmission volume) 23:00 ~22:00 Junction square Regulating tank Concentration tank Pre-stage filter basin BAC basin Post-stage filter basin Pattern Operating time zone 9:00 ~17:00 9:00 ~17:00 9:00 ~17:00 7:00 ~12:30 7:00 ~18:30 5:00 ~6:25 Operating time 24:00 8:00 8:00 8:00 5:30 11:30 1:25 Total volume of returned water (m 3 (24basins) (20basins) (4basins) ) Hourly volume of returned water (m 3 /h) Monday Pattern-A Tuesday Pattern-B Wednesday Thursday Pattern-C Friday Saturday Sunday Pattern-A Pattern-B Pattern-C Time zone Fig. 11: Predicted change of total manganese concentration in receiving well with time Dissolved manganese concentration (mg/l) Pattern-A Pattern-B Pattern-C Time zone Fig.12: Predicted change of dissolved manganese in receiving well with time

7 1-5 Manganese concentration in sludge The only way manganese can be discharged (out) from the plant is through sludge. The result of examination shows that the manganese content in the sludge of cake yard in wastewater treatment plant and in the drawn-out sludge from settling basin is 4 mg per 1 g of sludge. Table-2 shows the actual manganese inflow in raw water and the sludge for FY Table-2: Actual manganese inflow in raw water and sludge for FY2005: Raw water Raw water transmission volume (m 3 ) Total manganese (g/m 3 ) Inflow manganese (t) Dry weight (t) Sludge Manganese content (kg/t) Manganese discharge (t) 365,184, Diagram of form-wise manganese balance and circulation Based on the examination results of 1.1 ~ 1.5, the balance and circulation of manganese for one day in Asaka Water Purification Plant was estimated as shown in Fig The suspended manganese (155 kg) in receiving well is the sum total of the suspended manganese in Arakawa (40 kg), in returned water from washwater drainage system (105 kg) and wastewater treatment system (10 kg), while the dissolved manganese (90 kg) is the sum total of the dissolved manganese in Arakawa (50 kg) and in the returned water from wastewater treatment system (40 kg). 2Manganese from receiving well is divided approximately 20% to normal treatment system and 80% to advanced treatment system. 3 Normal treatment system The suspended manganese (30 kg) mostly gets settled down. The dissolved manganese (20 kg) gets oxidized through prechlorination, and more than 50% gets settled down as suspended manganese. After settling the suspended manganese (40 kg) is shifted to wastewater treatment system together with the sludge. Consequently, the suspended manganese that fails to get settled down and the dissolved manganese are arrested in the filter basin before being fed back as washwater drainage (10 kg). 4 Advanced treatment system Most of the suspended manganese (125 kg) gets settled down, with a small part getting arrested in the pre-stage filter basin before being fed back as washwater drainage (15 kg). The settled suspended manganese (100 kg) is shifted to the wastewater treatment system together with the sludge. The dissolved manganese (70 kg) passes through the settling basin and pre-stage filter basin, and mostly gets oxidized by ozone before being mostly arrested in the BAC basin, and is finally fed back as washwater drainage (75 kg). The dissolved manganese not arrested in the BAC basin gets oxidized by

8 ashw10kgwaterdrainagintermediate chlorination and is arrested in the post-stage filter basin before being fed back as washwater drainage (5 kg). 5 Wastewater treatment system Of the manganese discharged together with the sludge from normal treatment system (40 kg) and advanced treatment system (100 kg), approximately 35% (50 kg) is returned to the receiving well as suspended manganese (10 kg) and as dissolved manganese (40 kg), with about 65% (90 kg) discharged as sludge. Relation with sludge: The record in Table-2 shows that 8650 t of sludge is discharged annually, with the average daily discharge being 24 t. Supposing the manganese content in 24 t of sludge being 4.0 kg/t, the manganese contained in the sludge comes to be 96 kg, equivalent to the aforesaid sludge of approximately 65% (90 kg). The balance and circulation of manganese are estimated to be as mentioned above, and manganese can be broadly divided into suspended manganese and dissolved manganese. The suspended manganese mostly gets treated in settling basin, while the dissolved manganese gets oxidized by chlorine and ozone before being treated in settling basin, BAC basin and filter basin. On the other hand, the suspended manganese discharged together with the sludge from the settling basin gets dissolved under the anaerobic environment, and the dissolved manganese is again returned to the receiving well. In this way, manganese repeats dissolution and oxidization to circulate in the treatment systems, and the equivalent of manganese flowing from the river gets mixed in the sludge before finally being discharged out of the treatment systems. From Arakawa River Raw water 40 kg / 50 kg Suspended manganese / Dissolved manganese With underline : Measured value Without underline : Estimated value Receiving well 155 kg / 90 kg Returned water 10 kg / 40 kg Sulfuric acid, Pre-chlorination, PAC Intermediate chlorination, Post-PAC Post-chlorination, Caustic soda 40kg Normal treatment system 30 kg / 20 kg Settling basin Filter basin Treated water 105kg 15kg 75kg 5kg Wastewater treatment system Advanced treatment system 125 kg / 70 kg Sulfuric acid, PAC 100kg 140kg Settling basin Pre-stage filter basin ewaste water BAC basin Post-stage filter basin Treated water Ozone treatment plant Sludge 90kg Intermediate chlorination, Post-PAC Post-chlorination, Caustic soda Fig. 13 Balance and circulation diagram for one-day form-wise manganese

9 2. Countermeasures against manganese 2-1 Countermeasures using injection of chemicals: The following countermeasures were temporarily taken as the manganese concentration levels in the water treatment systems rose. <Advanced treatment system> Dissolved ozone management target value increased from 0.07 mg/l to 0.10 mg/l. Reinforced injection of intermediate chlorination and post-pac <Normal treatment system> From pre-weak intermediate chlorination to prechlorination 2-2 Change in frequency of sludge discharge: The sludge discharge from the settling basin had been carried out 3 times per week since August 2004, which is estimated to have caused the suspended manganese in the sludge accumulated in the settling tank to get reduced due to oxygen deficiency, etc., get settled down and dissolved in water. As a countermeasure, the sludge discharge has been carried out 5 times per week since April 2005 to carry out wastewater treatment before the manganese gets settled down and dissolved in water. In addition to this, the operation schedule was changed to shorten the detention time in regulating tank and concentration tank. 2-3 ph adjustment in concentration tank: Oxygen deficiency in regulating tank and concentration tank causes the suspended manganese to get reduced, resulting in the increase of dissolved manganese. In order to prevent this, table experiment was conducted on a method to reduce the dissolved manganese by adding alkali agent to the sludge for an alkalinity of ph 9, 10, 11. The result showed that two days after the ph adjustment, manganese with high concentration (24 mg/l) was found dissolved in the un-adjusted supernatant water, and when adjusted to ph 9 ~ 11, the manganese dissolution was scanty (3 mg/l or under). However, the higher the ph, the higher the generation of THM, with the values being 36, 46 and 69 μg/l respectively for ph 9, 10 and 11 against 32 μg/l when no ph adjustment is made. This is probably attributed to the fact that the higher the ph value, the greater the dissolution of humic substance in the sludge. 2-4 Addition of chlorine to dehydrated filtrate: Table experiment was conducted on the dehydrated filtrate in the returned water from wastewater treatment system with highest level of dissolved manganese concentration to see the possibility of dissolved manganese getting oxidized by adding chlorine. The result indicated that in the contact time of 30 minutes the dissolved manganese (17 mg/l) showed no change until chlorination dosage of 30 mg/l, but got reduced to half (9 mg/l) at chlorination dosage of 50 mg/l and to almost nil at 80 mg/l. However, the result also showed that at chlorination dosage of 80 mg/l, the free residual chlorine concentration level in treated water remains high at 40 mg/l.

10 Conclusions: The manganese concentration in raw water of Asaka Water Purification Plant is high at 0.1 mg/l, with 60% being the dissolved manganese. The dissolved manganese reached the post-stage filter basin, causing brownish-red bubbles to appear on the water surface in the post-stage filter basin. The manganese concentration in washwater drainage system is extremely high at 1 ~ 13 mg/l, with the load particularly from BAC basin being high. Most of the manganese in the washwater drainage is suspended manganese. Solid-liquid separated water from washwater treatment system is returned to the receiving well as the returned water. The manganese concentration is high in the order of the dehydrated filtrate in pressure dehydrator (17 mg/l) and the supernatant water in concentration basin (5.5 mg/l). The dissolved manganese content in returned water is 74%, covering more than half of the manganese content. The manganese content in the sludge is 4 g per 1 kg of sludge. The manganese content equivalent to the manganese from Arakawa is discharged together with the sludge. In order to prevent the appearance of brownish-red bubbles in the post-stage filter basin, countermeasure against the dissolved manganese in the returned water from wastewater treatment system is currently under consideration.