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 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 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 c Expected values Conventional processes 4 to 15 days c < 4 d: floc is not dense enough to settle c > 15 d: floc is too small and does not settle
25 Solids retention time 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 c, Y, k d and U Mass balance around the WWTP:
32 Relating c, Y, k d and U Assuming steady state and neglecting the influent active volatile solids: but then Full equation:
33 Relating 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.
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 4.500 mg/l
44 Aeration systems Pure oxygen oxygen is injected directly into the liquid phase oxygen conc