Secondary Treatment Process Control

Transcription

1 SARBS One-Day Training Seminar Phoenix Club, Anaheim Secondary Treatment Process Control Graham Juby June 5, 2013

2 Objectives Provide an understanding of nitrogen removal process interactions to support decision making for plant operations and process control. - and - Perhaps answer the question why are we taking and analyzing all these *!#@X* samples??? 2

3 Overview of Presentation Background to Activated Sludge process and nitrogen removal Typical operational and control challenges 3

4 Secondary treatment wastewater plants come in various shapes and sizes 4

5 But, their objective is the same: meet permit => remove BOD, TSS and nitrogen (and P) Raw WW MLR Prim Eff Ax Ox Eff Side Streams RAS WAS 5

6 Achieving Permit Compliance BOD 5 (COD) (TOC) TSS Nitrogen Parameter Character/Approach Organic material (carbonaceous) Heterotrophic bacteria Carbon from organic compounds Require oxygen Produce CO 2 Aerobic conditions Good settling sludge No pin-flocs Organic Inorganic 6

7 Total nitrogen has several components Organic Nitrogen Inorganic Nitrogen Particulate Organic - N Soluble Organic - N Ammonia - N Nitrite - N Nitrate - N Total Kjeldahl Nitrogen (TKN) NO X -N Total Nitrogen 7

8 Achieving Permit Compliance Parameter Character/Approach BOD 5 (COD) (TOC) Organic material (carbonaceous) Heterotrophic t bacteria Carbon from organic compounds Require oxygen Produce CO 2 Aerobic conditions TSS Good settling sludge No pin-flocs Nitrogen Organic Inorganic Ammonia Inorganic Oxidized by chemoauotrophic bacteria (ammonia oxidizing bacteria) Energy from chemical reaction Require oxygen Carbon from CO 2 Aerobic conditions 8

9 Nitrogen removal involves two biological reactions Step 1 Ammonia Nitrification (aerobic conditions) Ammonia oxidizing bacteria Ammonia (NH 3 ) Nitrite (NO 2 ) Nitrate t (NO 3 ) NH O 2 NO 3 + H 2 O + H + 9

10 Nitrogen removal involves two biological reactions Step 2 Denitrification (under Anoxic conditions) Nitrate t (NO 3 ) Nitrogen Gas (N 2 ) De-nitrifiers (CH H +CO+OH - 2 O) n + NO 0.5H 2 O N OH Carbon Source 10

11 Nitrification requires more process oxygen Nitrification For every milligram of ammonia oxidized to nitrate, 4.6 milligrams of oxygen are consumed 30 mg/l; 10-mgd flow = 2,502 lb/d NH3-N» 11,500 lb/d oxygen Note: If there isn t enough biomass (solids inventory) in the aeration basin, adding more DO will not remove more ammonia Denitrification (Anoxic Zone) De-nitrifiers preferentially consume oxygen over nitrate, t reducing nitrate t conversion efficiency i ~ 60% oxygen recovered 11

12 Nitrification consumes alkalinity, but de- nitrification gets some back Nitrification For every pound of ammonia oxidized to nitrate, about 7.1 pounds (as CaCO 3 3) is consumed Nitrifier s growth rate impacted if ph drops below ~ 7.0 Denitrification Approximately 3.0 to 3.6 pounds (as CaCO 3 ) is produced per pound of nitrate converted 40-50% alkalinity li it recovered 12

13 Monitoring NO2 - gives an indication of completeness of nitrification Ammonia (NH 3 ) Nitrite (NO 2 - ) Nitrate (NO 3 - ) 1 mg/l of ammonia consumes 7.6 mg/l of chlorine And, 1 mg/l of nitrite (NO2 - ) consumes 5 mg/l of chlorine 13

14 Presence of ammonia and NO - 2 can cause disinfection issues 20 NH4 N NO2 N 18 NO3 NN TIN Linear (NO3 N) Linear (TIN) /01/12 07/02/12 08/02/12 09/02/12 10/03/12 11/03/12 14

15 Understanding process balances Nitrification Nitrifying bacteria in solids inventory Denitrification Denitrifying bacteria in solids inventory Oxygen Alkalinity No Oxygen BOD 15

16 Typical nitrogen removal process configuration Raw WW MLR Prim Eff Ax Ox Eff Side Streams RAS WAS 16

17 Water temperature impacts nitrification efficiency Nitrifier Growth Rate o C o F Maximum rate Fall to near zero (fails) Proceeds at slower rate > 40 > 104 < 20 < 68 E F F I C I E N C Y o C 17

18 Activated sludge capacity determined by several factors SRT= 2 days SRT= 5 days SVI = 150 ml/g mgd Capacity, ,000 2,000 3,000 4,000 5,000 6,000 MLSS concentration, mg/l 18

19 Activated sludge capacity determined by several factors 70 mgd Capacity, SRT= 2 days SRT= 5 days SVI = 150 ml/g Aeration Capacity (example) ,000 2,000 3,000 4,000 5,000 6,000 MLSS concentration, mg/l 19

20 Typical challenges to nitrogen removal activated sludge systems 1. Solids Retention Time (SRT) 2. Dissolved Oxygen (DO) Control 3. Carbon Requirements 4. Anoxic zone mixing 5. Sidestream Management 20

21 SRT is the most important control parameter in an activated sludge system The SRT fixes: 1. The growth rate of the biomass and, therefore, sludge quality (how the sludge flocculates, settles, and compacts), 2. The extent of BOD 5 conversion, 3. Nitrification, and the effluent ammonia concentration 4. The MLSS concentration and, therefore, the biomass inventory of the system, and 5. To some extent, the oxygen required. 21

22 There are different ways to calculate the SRT Total SRT Typical calculation : (Volume of Bioreactors) X (MLSS) (Waste Flow) X (Waste TSS) + (Eff. TSS) X (Eff. Flow) Aerobic SRT = (Volume of Aerated Portion of Bioreactor) X (MLSS) (Waste Flow) X (Waste TSS) + (Eff. TSS) X (Eff. Flow) 22

23 Nitrification and Aerobic SRT o C Influent Ammonia (mg/l) Partial Nitrification Danger Zone No Nitrification 68 o F Colder temperature requires longer SRTs for comparable performance Complete Nitrification Aerobic SRT, days 23

24 WAS sampling times impact SRT calculation 7,000 6,000 5,000 WAS TSS S (mg/l) 4,000 3,000 2,000 Hourly Concentration Daily Average 1, Hour 24

25 Process control considerations: 1. SRT control takes constant t vigilance il 2. Regularly check data used to calculate daily WAS rates for accuracy and precision 3. Wild swings in actual SRT can occur, which will upset the process performance 4. Use 7-day moving averages for MLSS and WAS rate to reduces large swings in waste rates 25

26 Operational Tip: Consider tracking ratio of pounds of aerobic solids to pounds of influent ammonia Denitrifiers MLR Nitrifiers Solids in the bottom are not working Anoxic Aerobic RAS WAS 26

27 Operational Tip: Consider switching to a hydraulic calculation l of SRT Q WAS = Volume of AB * (Q RAS /Q FEED ) SRT * (Q RAS /Q FEED + 1) No TSS samples and analysis Need to account for losses over clarifiers Greenwood et al.,

28 DO Control Energy wastage Excess DO goes to downstream anoxic zones 28

29 DO Control offers potential for power savings Diu urnal Aerat tion Power (bhp) Power Savings No DO Control Actual Demand Hour 29

30 DO in MLR flow impacts denitrification MLR holds dissolved oxygen Spot measurement to confirm minimal DO prior to discharge to anoxic zone 30

31 Operational Tip: Check and calibrate DO probes regularly l 31

32 Carbon Requirements 32 Wave.pptx

33 Minimum carbon ratios determine efficiency of de-nitrification Source cbod 5 : TKN Henze et al., Randall et al WEF, Metcalf & Eddy,

34 Anoxic zone mixing impacts efficiency of denitrification Good contacting Avoid short circuiting iti 34

35 Simple checks can be done when something goes wrong Review assumptions (meeting targets, communications, formulas) Perform mass balances of components and confirm calculations Confirm that DO probes are operating and calibrated Confirm carbon : nutrient ratios Evaluate sampling procedures Inspect auto samplers for proper operation QA lab work 35

36 Daily proactive permit compliance Operational Targets Field Test Verify Compliance Predict Daily Performance Process Adjustment 36

37 Final point Activated t sludge is a living i biological i l system and it adapts to changes in its environment. Regular monitoring is needed to see how it s doing 37

38 SARBS One-Day Training Seminar Phoenix Club, Anaheim Secondary Treatment Process Control Graham Juby June 5, 2013

39 Sidestream Management 39 Wave.pptx

40 Common operating challenges Solids inventory control through accurate calculation inputs, representative sampling or understanding di the method Aeration control for nitrification Insufficient alkalinity for nitrification DO recycled to denitrification Insufficient carbon for denitrification Side stream management Delaying maintenance 40

41 Proactive approach leads to greater success Understand and manage Interactions within nutrient removal process Interaction between processes Effects from sidestreams Develop and trend ratios Review data and trends daily Anticipate and prepare for influent seasonal changes nutrients, temperature 41