Activated Sludge Plants: Dimensioning Eduardo Cleto Pires

2 History Seminal paper: Ardern, E. and Lockett, W.T. (1914) Experiments on the oxidation of sewage without the aid of filters, Journal Society of Chemical Industries, v.33, p.523 First experiments were performed in batch reactors keeping the biological sludge from batch to batch

3 History Continuous flow reactors soon substituted batch reactors using a settling tank for sludge separation and return. The process was “discovered” instead of “invented” as Arden & Lockett were investigating sewage treatment using published observations on the effect of oxygen on sewage and almost accidentally observed the effects of sludge recycling.

4 Why the denomination of “activated sludge”? Ardern & Lockett observed that the sludge improved (faster reaction) after a few batches as if “activated” by the recycling and aeration! Thus activated sludge Actually, a mixture of microorganisms formed macroscopic flocs and this diverse and concentrated population of microorganisms is responsible for the higher efficiency.

5 Definition Activated Sludge is the name given to the mixture of microorganisms organized as biological flocs formed by recirculation of the biomass from a separation device back into the aeration tank.

6 Pros and Cons Pros high treatment efficiency operation control and flexibility small foot print Cons needs precise control and operation needs frequent laboratory measurement of many variables higher costs

7 Activated Sludge Fundamentals

8 Activated sludge process kinetics

9 Substrate utilization rate – U The substrate utilization rate is the ratio of substrate removal velocity and the mass of microorganisms. U- substrate utilization rate [T -1 ] S- substrate concentration [ML -3 ] X av - volatile suspended solids at the aeration tank – MLVSS* [ML -3 ] t- time [T] * - mixed liquor volatile suspended solids

10 Substrate utilization rate – U Assuming constant U and solving for an interval equal to the hydraulic detention time (  H ): S 0 - influent substrate concentration S e - effluent substrate concentration Q- sewage flowrate V- aeration tank volume   - hydraulic detention time ( V / Q )

11 Michaelis-Menten kinetic equation U max - maximum substrate utilization rate S- substrate concentration at the aeration tank K S - half-velocity constant (substrate concentration when U is equal to U max

12 Properties of the Michaelis- Menten equation High substrate concentration: equation approaches zero order kinetics. Low substrate concentration: equation approaches first order kinetics.

13 Removal rate of BOD or COD Apply the Michaelis-Menten or first order kinetics equation being the COD or the BOD represented by S. Assuming first order kinetics (S ≡ BOD or COD): For sanitary sewage: K ranges from 0,017 to 0,030 d -1.

14 Mass balance around the aeration tank Mass balance assuming first order kinetics: V S e X av Aeration Tank Q S 0 Q S e

15 Mass balance around the aeration tank This equation is used to estimate the needed hydraulic detention time to reach the desired effluent concentration Simplifying and remembering that where K = kX av

16 Mass balance considering recirculation This is the same expression obtained without sludge return (recirculation). Thus the recirculation of the sludge has no effect on the mass balance (it is an internal process!).

17 Mass balance considering recirculation If recirculation has no effect on mass balance why is it used? To keep the biomass in the aeration tank! To uncouple the biomass retention time from the hydraulic retention time.

18 Mass balance considering recirculation Then, how could we avoid or reduce sludge recirculation? Using attached biomass (biomass carrier). suspended growth x attached growth

19 Food to Microorganisms ratio (F/M) This ratio is also known as food-mass ratio. Mass refers to the amount of microorganisms measured as the concentration as volatile suspended solids

20 Food / Microorganisms ratio (F/M) It is the ratio of the available food [ F ] (substrate) in the aeration tank and the quantity of microorganisms that will feed on this substrate (MLVSS) [ M ].

21 Food / Microorganisms ratio (F/M) Usual values (NBR 12209:2011) high rate F/M from 0,70 to 1,10 kgBOD/kgMLVSS conventional F/M from 0,20 to 0,70 kgBOD/kgMLVSS extended aeration F/M  0,15 kgBOD/kgMLVSS

22 Solids retention time – θ c Average time that a particle remains in aeration. Also known as: sludge age mean cell residence time Numerically it is equal to the mass of suspended solids at the aeration tank and the mass of wasted solids (excess sludge).

23 Solids retention time – θ c Q e - effluent flowrate (withdraw at the secondary clarifier) Q u - sludge flowrate (withdraw at the secondary clarifier) X av - volatile suspended concentration solids at the aeration tank X uv - volatile suspended solids concentration in the wasted sludge X ev - volatile suspended solids concentration in the treated wastewater

24 Solids retention time – θ c Expected values Conventional processes 4 to 15 days θ c < 4 d: floc is not dense enough to settle θ c > 15 d: floc is too small and does not settle

25 Solids retention time – θ c Controls nitrification: high  c favors nitrification Used as a control parameter: estimation of the sludge volume to be wasted

26 Synthesis and auto-oxidation Synthesis a fraction of the organic matter is synthesized into new cells the mass of microorganisms increases Endogenous respiration or auto- oxidation a fraction of the cells decays the mass of viable microorganisms decreases

27 Synthesis and auto-oxidation Degradation of the organic matter Organic Matter CO 2, H 2 O, N 2, P End products CO 2, H 2 O, NH 3, P Non-biodegradable end products New cells Energy Synthesis Endogenous respiration

28 Synthesis and auto-oxidation Synthesis fraction biomass yield Y

29 Synthesis and auto-oxidation Decay fraction (endogenous respiration) endogenous decay coefficient- k d

30 Synthesis and auto-oxidation Balance of cell mass at the aeration tank: Y- 0,40 to 0,50 mgSSV/mgBOD removed This is the produced sludge! k d - 0,05 to 0,10 (mgSSV/d)/mgSSV

31 Relating θ c, Y, k d and U Mass balance around the WWTP:

32 Relating θ c, Y, k d and U Assuming steady state and neglecting the influent active volatile solids: but then Full equation:

33 Relating θ c, Y, k d and U one develops other relations: Rearranging

34 Sludge production Net sludge yield - ∆X: The sludge yield is also expressed as:

35 Sludge production The net yielded sludge is the sludge that needs to be wasted. this sludge is digested in anaerobic reactors (large plants) prior to discharge or, in small plants, mixed with lime for chemical stabilization and discarded.

36 Relation between Y and Y obs Equating both equations for ∆X: or

37 Sludge recirculation Keeps a high and constant sludge concentration at the aeration tank. Inoculation of the aeration tank speeding the stabilization of the organic matter.

38 Sludge recirculation Recommended recirculation rates: MLVSS < 3500 mg/L25% 3500 < MLVSS < 4500 mg/L50% MLVSS > 4500 mg/L100%

39 Oxygen requirements Oxygen is consumed to provide energy for synthesis of new cells endogenous respiration Injection of oxygen (air) provides mixing in the aeration tank, keeping the flocs suspended stripping (removal) of volatile compounds, formed as metabolites or existing in the polluted water

40 Oxygen requirements Required mass of oxygen: Thus

41 Oxygen requirements – design criteria Approximate values for a' and b’ a’≈ 0,52 b’≈ 0,12 d -1 Minimum oxygen concentration at the aeration tank 1,5 a 2 mgO 2 /L

42 Choosing an aeration system Consider shape of the aeration tank mixing requirements cost operation

43 Aeration systems Conventional air is injected into the liquid phase in the aeration tank and oxygen from the air transfers to the water diffused air mechanical mixing mixed systems (diffused air + mechanical mixing) conventional systems are useful for biomass concentration up to mg/l

44 Aeration systems Pure oxygen oxygen is injected directly into the liquid phase oxygen concentrator liquid oxygen tank pure oxygen requires specific equipment for injection high biomass concentration, up to mg/l low hydraulic retention time: 3 hr!

Surface aeration 45

46 Surface aeration Maintenance: many units to keep running

Bubbling aeration 47

48 Fine bubble diffusers

49 Distribution of diffusers in an aeration tank

50

Micro holes (micro pore) pipe diffuser 51

52 Dimensioning of the aeration system Diffused air air pressure needs to surpass water column (static pressure) pipeline pressure drop pressure drop at the diffusers

53 Dimensioning of the aeration system Diffused air blowers (compressors) centrifugal blowers flowrate above 30 m 3 /min and pressure from 5 to 7 MWC positive displacement blowers flowrate below 30 m 3 /min and pressure above 6 MWC

54 Power requirement P- blower power [kW] M air - required air mass [kg/s] Q air - air flow rate [m 3 /s] R- gas constant for air [8,314 kJ/kmol.K] 8,41- air constant [kg/kmol] – adjustment of units T 0 - inlet absolute temperature of the air [K] p e - absolute pressure at the blower inlet [atm] p s - absolute pressure at the blower outlet [atm]  - blower efficiency [0,70 to 0,80]

55 Recommendations first estimate of pressure drop 1.2 to 1.5 water column blowers should be capable do deliver 1.5 times the required air flow always have a spare blower

56 Dimensioning of surface aerators Surface aerators Required power is estimated using manufacturer’s data oxygen transfer rate as a function of power: standard conditions (sea level at 20°C and tap water) the standard condition values are corrected to field conditions: altitude, temperature, sewage characteristics Besides oxygen transfer it is necessary to consider mixing (suspension of the solid phase)

57 Correcting to field conditions N- oxygen mass transferred under field conditions N 0 - oxygen mass transferred under standard conditions C SW - oxygen saturation concentration in the aeration tank at temperature T - assumed as 95% of the tap water saturation concentration C L - oxygen concentration in the saturation tank - usually the minimum value is 2.0 mgO 2 /l  - correction coefficient to take into account industrial wastewater mixed with the sanitary sewage - usual values are in the range 0.8 to 0.9 NOTE: some constant values may differ depending on the author or country standards.

58 Secondary clarifiers The quality of the secondary clarifier is fundamental to assure the operation of activated sludge plants. Dimensioning depends on sludge settleability and standard design procedures and dimensions. Dimensions used should be the ones that provide the highest safety factors.

59 Nitrogen removal with denitrification Clarifier Recirculation Anoxic tank BOD removal Denitrification Aeration tank BOD removal Nitrification Influent Effluent Wasted sludge

60 Nitrogen removal with denitrification Oxidation Ditch

61 References Tchobanoglous, G.; Burton, F.L. and Stensel, H.D. Wastewater Engineering Treatement and Reuse (Metcalf & Eddy). McGraw Hill, 4 th. ed., 2003 (Chap. 7 and 8)