Activated Sludge Process Control:

Transcription

1 2015 Pacific Water Conference Activated Sludge Process Control: What s Important and How It s Easily Implemented February 2015

2 Presentation takeaways 1. MCRT/SRT is the only way to control the activated sludge process 2. MCRT/SRT controls whether a plant will nitrify and sludge quality (how the sludge flocculates, settles and compacts) 3. Wasting mixed liquor rather than RAS greatly simplifies MCRT/SRT control, but beware the recycle flow 4. NEORSD is constructing a wasting clarifier and it s going to be awesome 2

3 NEORSD Southerly Wastewater Treatment Center two stage activated sludge 3

4 NEORSD Southerly Wastewater TreatmentCenter first stage improvements FST 1-6 FST 11 Normal Flow to Second Stage Lift Station FSTs 7-10 Future Disinfection and Pump Station Discharge N 4

5 Existing FST Configurations FSTs Nos. 1-6 FSTs Nos Circular Gould Type II Rectangular 110 Dimensions (ft) 110 x Side Water Depth (ft) 12.6 No Energy Dissipating Inlet NA 937 Weir Length (ft) foot dia. x 5 foot deep floc well Perforated Baffle Wall in-board and peripheral launders / dual weirs Launders 8 x 144-foot long dual-weir length launders Organ pipe hydraulic suction removal Collector Type (2) Traveling bridges with hydraulic siphon 5.7 RAS per tank (MGD)

6 One of the most useful equations operators and engineers will ever know SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a MLSS concentration

7 One of the most useful equations operators and engineers will ever know SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a Solids residence time (also MCRT)

8 One of the most useful equations operators and engineers will ever know SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a Influent flow

9 One of the most useful equations operators and engineers will ever know SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a On-line aeration basin volume

10 One of the most useful equations operators and engineers will ever know SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a Influent inorganic TSS concentration

11 One of the most useful equations operators and engineers will ever know SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a Yield

12 One of the most useful equations operators and engineers will ever know SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a Decay rate

13 One of the most useful equations operators and engineers will ever know SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a Influent total BOD concentration

14 One of the most useful equations operators and engineers will ever know SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a Secondary effluent soluble BOD concentration

15 What is in the control of operators? SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a Pretty much only the SRT (controlled by WAS rate)

16 Most commonly used strategies for WAS control 1. Control by constant MLSS or MLVSS concentration 2. Control by constant MLSS or MLVSS mass inventory 3. Control by constant F:M ratio 4. Control by constant SRT, MCRT, sludge age 16

17 This equation can be used to show that: SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a 1. The MLSS concentration is a response variable, so cannot be used as a control variable

18 This equation can be used to show that: SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a 2. The MLSS mass is a response variable, so cannot be used as a control variable

19 This equation can be used to show that: SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a 3. The MLVSS concentration is a response variable, so cannot be used as a control variable

20 This equation can be used to show that: SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a 4. The MLVSS mass is a response variable, so cannot be used as a control variable

21 This equation can be used to show that: SRT Q BOD MLSS = (ISS INF + Y g INF sbod EFF ) 1 + b SRT V a 5. The F:M ratio is a response variable, so cannot be used as a control variable

22 Another look (this is REALLY important) MLSS V a = SRT Q (ISS INF + Y g ) BOD INF sbod EFF 1 + b SRT My own experience

23 SRT is the ONLY way, and by the way, the other techniques don t work

24 Only one matters, forget the others 1. Control by constant MLSS or MLVSS concentration 2. Control by constant MLSS or MLVSS mass inventory 3. Control by constant F:M ratio 4. Control by constant SRT, MCRT, sludge age 24

25 Did I mention it doesn t work

26 The prospect of direct discharge from Southerly s first stage a game changer 1. Consistent effluent quality a must and, therefore, so is sludge quality. 2. Need to prevent nitrification. 26

27 Growth rate, G R, controlled by SRT G R = (1/SRT) + γ+ b Because γ and b are small G R (1/SRT) 27

28 Controlling growth rate in activated sludge is paramount 1. Controls sludge quality: How the sludge flocculates, settles and compacts 2. Controls whether or not nitrification will occur 28

29 SRT controls sludge (and effluent) quality and the number of rotifers

30 Either nitrifiersare in the mixed liqouror they re not; SRT controls

31 Calculating MCRT can be tedious MCRT = V a MLSS+ V blanket TSS blanket Q WAS TSS WAS + Q EFF TSS EFF

32 Simplifies if V blanket 0 and TSS EFF < 10 mg/l ( 0) SRT = V a MLSS Q WAS TSS WAS

33 Waste mixed liquor instead of RAS (TSS WAS = MLSS) SRT = V a MLSS Q WAS MLSS

34 Wow! Greatly Simplified! SRT = V a Q WAS If this is so easy, why don t we do it more often?

35 For the simple reason that the RAS is more concentrated TSS RAS = ( Q Q RAS + 1) MLSS For example, if Q RAS = 75% of Q MLSS = 2,000 mg/l, then TSS RAS = 4,700 mg/l

36 Rearranging equation to determine Q WAS needed for target SRT Q WAS = V a SRT target For example, if a 2-day SRT is targeted, this equation says that Q WAS has to be set equal to 1/2 the on-line aeration basin volume per day.

37 The wasting clarifier is born! N Existing Excess (Waste) Activated Sludge (EAS) FST 11 Aeration Tanks FST No. 1-6 Control ML flow to FST 5: Waste all settled solids from this FST only Existing EAS Forceman Gravity Belt Thickeners 37

38 SRT control is easy and elegant 1. How many N aeration basins are on line? 2. What SRT is needed to not nitrify and give best sludge and effluent quality. Existing Excess (Waste) Activated Sludge (EAS) FST Set mixed liquor flow to Va/SRTtarget to wasting clarifier (FST No. 5). Aeration Tanks FST No Waste all solids in underflow from wasting clarifier to GBTs Existing EAS Forceman 5. Return all solids from other FSTs to aeration basins; maintain low blankets Gravity Belt Thickeners 38

39 Opportunity for GBT optimization as well N Existing Excess (Waste) Activated Sludge (EAS) FST 11 Aeration Tanks FST No Waste all solids in underflow from wasting clarifier to GBTs Existing EAS Forceman Gravity Belt Thickeners 39

40 Presentation takeaways MCRT/SRT is the only way to control the activated sludge process MCRT/SRT controls whether a plant will nitrify and sludge quality (how the sludge flocculates, settles and compacts) Wasting mixed liquor rather than RAS greatly simplifies MCRT/SRT control, but beware the recycle flow NEORSD is constructing a wasting clarifier and it s going to be awesome 40

41 Thank you!