Key Points. The Importance of MCRT/SRT for Activated Sludge Control. Other (Confusing) Definitions. Definitions of SRT

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1 Key Points The Importance of MCRT/SRT for Activated Sludge Control Randal Samstag Carollo Engineers Definitions Why is SRT important? Examples of use of SRT for process control Nitrification control Settleability control Control of bioflocculation Definitions of SRT Solids Residence Time (SRT): The average time treatment organisms spend in the activated sludge reactor. Aerobic SRT: The average time treatment t t organisms spend in the aerated part of the activated sludge reactor. Total SRT: The average time treatment organisms spend in the activated sludge process. Other (Confusing) Definitions Sludge age: The theoretical length of time activated sludge remains in the process. (WPCF Manual of Practice No., Operation of Wastewater Treatment Plants) Mean cell residence time (MCRT): An expression of the average time that a microorganism or solid particle undergoes aeration ation in the activated ated sludge process. (State of Pennsylvania, Earth Wise Academy Website) Solids retention time (SRT): An expression of the average time that a microorganism or solid particle undergoes aeration in the activated sludge process. (State of Pennsylvania, Earth Wise Academy Website)

2 Activated Sludge Schematic Flow Stream Abbreviations Q IN X IN Q MLSS S IN Reactor Tank Anaerobic or Anoxic Aerobic V AB X MLSS Sedimentation Tank V SC X Q Q R X R Q W X W S Q: Flow (mgd) V: Volume (mgal) X: Suspended Solids Concentration (mg/l) S: Biochemical Oxygen Demand (mg/l) IN: Influent : Effluent MLSS: Mixed Liquor Suspended Solids R: Return Activated Sludge W: Waste Activated Sludge AB: Aeration Reactor Tank SC: Secondary Clarifier / Sedimentation Tank Mathematical Definition of SRT But What Mass and What Waste? Mass( pounds) SRT( days) Waste( pounds/ day) Mass ( Pounds ) Volume ( mgal )* Concentration ( mg / L ) Waste( pounds/ day) Flow( mgal/ day)* Concentration( mg / L) For Total SRT: The total mass and the total waste The total mass is the mass in the aeration reactor and the secondary sedimentation tank The total waste is the waste that leaves with the effluent and the waste that leaves in the waste activated sludge

3 Mathematical Definitions Mass( pounds) SRT( days) Waste( pounds/ day) V SRT ( days ) Q AB WAS MLSS WAS MAX* S SRT K S S V Q SC kd SC How Do You Measure The Average Concentration In The Secondary Clarifier? Use a sludge judge: Take a full depth sample Empty the whole tube contents into a gallon bucket Mix it up and take a sample of the mixed sludge Just to Make Things Confusing Why is SRT Important? We (engineers) often define a reactor SRT based on just the solids in the activated sludge reactor: Because: It controls the kind of organisms that can survive in the process. SRT( days) Q WAS V AB WAS Q MLSS

4 Another Definition Growth Rate Curves for Heterotrophs Washout SRT: The SRT below which a given organism will not grow fast enough to be maintained in the system. It will be washed out of the system. Growth Rate (/day)... Growth Rate Curves Zooglea Nocardia S. Natans PAO C. Denitrificans Growth Rate Curves for Slower Growing Organisms Washout SRT for Different Organisms. Growth Rate Curves Ammonia Oxidizers Nitrite Oxidizers Methane Formers Zooglea S. Natans PAO C. Denitrificans Nocardia Ammonia Oxidizers Nitrite Oxidizers Methane Formers Growth Rate (/day) Washout SRT (Days)

5 Examples of use of SRT for process control Nitrification control Settleability control Control of bioflocculation Nitrification Control Nitrifiers convert ammonia to nitrate Nitrifiers are slow growing organisms that will be washed out of the system unless the SRT is greater than to days (depending on the temperature.) Washout SRT for Nitrifiers Dependence of Washout SRT on Temperature Ammonia Oxidizers Nitrite Oxidizers Washout SRT for Ammonia Oxidizers Temperature = deg C Temperature = deg C Temperature = deg C No Nitrification Zone of unstable operation. Stable Nitrification Washout SRT (Days) Washout SRT (Days)

6 Dependence of Washout SRT on Dissolved Oxygen Concentration SRT Control of Settleability Washout SRT for Ammonia Oxidizers DO =. mg/l DO =. mg/l DO =. mg/l. Low SRT can cause outbreaks of Type 86 High SRT encourages Nocardioforms and M. parvicella Bio P organisms wash out below days SRT Nitrifiers wash out below - days SRT Washout SRT (Days) Typical SRT for Filaments Name Mug Shot Typical SRT Type 86 Under days Microthrix Parvicella Greater than days Nocardia Greater than Days Control of Settleability with Selectors Three kinds of selectors Aerobic Anoxic Anaerobic SRT is key to success of all Sphaerotilus natans to 7 Days Type 8 days 6

7 Selector A Definition Metabolic Control Influent Selector Main Basin Clarifier Effluent A tank upstream of the main aerobic portion of the activated sludge process that is designed to control sludge settleability by metabolic or kinetic means Metabolic control Due to the way the organisms get food and energy Kinetic control Due to the growth rate of the organism under different conditions of growth (SRT) Designed to encourage a certain organism by providing the right metabolic conditions for its growth Examples PAO in anaerobic selectors Anoxic selectors for S. natans and others Kinetic Theory of Selection (Chudoba, 97; Jenkins, 97) Filaments have a competitive advantage over floc forming organisms under conditions of low substrate (food) concentration gradient (change). Selectors work by exposing treatment organisms to a high substrate concentration gradient. Comparative Growth Rates DO. mg/l, Degrees C Spec Growth Rate, 7 6 Zooglea ramigera Sphaerotilus natans Type N 6 Substrate, mg/l 7

8 Comparative Growth Rates DO mg/l, Degrees C Comparative Growth Rates DO. mg/l, Degrees C Rate, Spec Growth Zooglea ramigera Sphaerotilus natans N 6 Rate, Spec Growth... Zooglea ramigera Sphaerotilus natans N 6 Substrate, mg/l Substrate, mg/l Aerobic Selector Aerobic Selector Influent Selector Main Basin Clarifier Effluent Selector BOD Main Basin CO + H O O Aerobic first stage Classic kinetic mechanism SRT or more days Relies on higher substrate concentration in smaller first stage of treatment BOD CO + H O O Synthesis 8

9 Anoxic Selector Anoxic Selector Influent Selector Main Basin Clarifier Effluent Selector BOD Main Basin CO + H O O Anoxic first stage Denitrification flow scheme Internal recycle Must nitrify! SRT - to days Most filaments don t denitrify Does not control M. Parvicella, which can denitrify BOD NO N CO + H O Synthesis Anaerobic Selector Anaerobic Selector PAO Phosphorus Accumulating Organisms Influent Selector Main Basin Clarifier Effluent Selector BOD Main Basin CO + H O O Anaerobic first stage Encourage PAO and/or GAO SRT -. days to days No internal recycle required Can encourage Thiothrix if it produces H S PAO need both anaerobic and fully aerobic conditions Glycogen Poly P Reducing Power PO Glycogen Synthesis Poly P PO 9

10 Influence of SRT and Temperature on PAO (Jenkins) T (Days) Washout SRT 8 6 Nitrifier Washout Nocardia Washout PAO Washout EBPR EBPR and Nitrification No EBPR Temperature (Degrees C) Flocculation of Biological Solids Flocculation is a key element in the settling process as biological particles are individually too small to settle out effectively in the secondary clarifier; flocculation is what enables them to form larger settleable flocs that can settle out. In suspended growth systems, flocculation is facilitated by an exocellular biopolymer. Activated Sludge Floc Exocellular Biopolymer Polysaccharides + Proteins Filaments Bacterial Colony Biopolymer Production is a Function of SRT and Temperature Supported by Data Showing Impact of SRT on Secondary Effluent PSD Biopolym mer T = deg C SRT T = deg C Need a higher SRT at lower temperatures to initiate and maintain biopolymer production annesl, counts/ml Number of particles per size ch SE PSDs from Activated Sludge Plants Operating at SRTs in days SE PSDs from TF/SC and AS Plants Operating at SRTs < day SE Turbidity Ranging from NTU SE Turbidity Ranging from 7 NTU Particle diameter, mm

11 Data from Operating Plant Showing Effect of Increase in SRT Percent Finer (%) IMPACT OF SRT ON SE PSD SRT=. days SRT=. days Particle Size (m) Conclusions SRT is the most important process design and control parameter for the activated sludge process SRT controls the kind of organisms that can thrive in the system SRT control is key to many important issues, including: Nitrification Settleability control Bioflocculation Thank you! Questions?