Fundamentals of Biological Treatment

1. OVERVIEW OF BIOLOGICAL WASTEWATER TREATMENT Objectives of Biological Treatment transform(i.e., oxidize) dissolved and particulate biodegradable constituents into acceptable end products (2) capture and incorporate suspended and nonsettleable colloidal solids into a biological floc or bio-film (3) transform or remove nutrients, such as nitrogen and phosphorus (4) in some cases, remove specific trace organic constituents and compounds Role of Microorganisms in Wastewater Treatment

Figure Typical(simplified) flow diagrams for biological processes used for wastewater treatment: (a) activated-sludge process, (b)aerated lagoons, (c) trickling filters, and (d) rotating biological contactors.

Types of Biological Processes for Wastewater Treatment suspended growth and attached growth(or bio-film) processes

Suspended Growth Processes

Attached Growth Processes

2. COMPOSITION AND CLASSIFICATION OF MICROORGANISMS Cell components (2) cell composition (3) environmental factors that affect microbial activity (4) methods used to identify and classify microorganisms Cell Components

Cell Composition 80% : water 20% : dry material 90% : organics 10% : inorganics

3. INTRODUCTION TO MICROBIAL METABOLISM Carbon and Energy Sources for Microbial Growth Carbon and Energy Sources : substrates Carbon Sources : heterotrops : organic carbon new biomass autotrops : carbondioxide cell carbon Energy Sources : phototrops : light energy source chemotrops : carbondioxide cell carbon

Nutrient and Growth Factor Requirements Principal inorganic nutrients : N, S, P, K, Mg, Ca, Fe, Na, and Cl Minor nutrients : Zn, Mn, Mo, Se, Co, Cu, and Ni Required organic nutrients, known as growth factors, are compounds needed by an organism as precursors or constituents of organic cell material, which cannot be synthesized from other carbon sources. Although growth factor requirements differ from one organism to another, the major growth factors fall into the following three classes:(1) amino acids,(2) nitrogen bases (i.e., purines and pyrimidines),and (3) vitamins.

4. BACTERIAL GROWTH AND ENERGETICS bacterial reproduction bacterial growth patterns in a batch reactor (3) bacterial growth and biomass yield (4) methods used to measure biomass growth (5) estimating cell yield and oxygen requirements from stoichiometry (6) estimating cell yield from bioenergetics (7) observed versus synthesis yield O2 Orgainics (BOD) Bacteria (C5H7O2N) O2 O2 End product CO2, NH3 etc.

Bacterial Growth Patterns in a Batch Reactor The lag phase The exponential-growth phase The stationary phase The death phase

Bacterial Growth and Biomass Yield The ratio of the amount of bio-mass produced to the amount of substrate consumed (g biomass/g substrate) for nitrification the yield : g-biomass/g-NH4-N oxidized for the anaerobic degradation of volatile fatty acids (VFAs) to produce methane : g-biomass/g-VFA used G-biomass/g-COD removed or g-biomass/g-BOD removed Measuring Biomass Growth * volatile suspended solids (VSS) or particulate COD(total COD minus soluble COD) * protein content, DNA, and ATP, a cellular compound involved in energy transfer

Estimating Biomass Yield and Oxygen Requirements from Stoichiometry ThOD

The quantity of oxygen utilized the oxygen used for substrate oxidation to CO2 and H2O the COD of the biomass the COD of any substrate not degraded the oxygen consumed per unit of COD used

Estimating Biomass Yield from Bioenergetics

Stoichiometry of Biological Reactions

Biomass Synthesis Yields for Different Growth Conditions

5. MICROBIAL GROWTH KINETICS microbial growth kinetics terminology Rate of utilization of soluble substrate (3) other rate expressions for the utilization of soluble substrate (4) rate of soluble substrate production from biodegradable particulate organic matter (5) the rate of biomass growth with soluble substrates (6) kinetic coefficients for substrate utilization and biomass growth (7) the rate of oxygen uptake (8) effects of temperature (9) total volatile suspended solids and active biomass (10) net biomass and observed yield.

Microbial Growth Kinetics Terminology

Other Rate Expressions for the Utilization of Soluble Substrate Rate of Soluble Substrate Production from Biodegradable Particulate Organic Matter

Rate of Biomass Growth with Soluble Substrates

Kinetic Coefficients for Substrate Utilization and Biomass Growth

Determination of Kinetic coefficients Laboratory reactor used for the conduct anaerobic treatment studies. Bench-scale continuous flow stirred reactors used for the determination of kinetic coefficients (a) without recycle (b) with recycle

Intercept Slope

Determination of kinetic coefficients

Y = 0.5 μmax = kY = 3.125 × 0.5 = 1.563 day-1

Rate of Oxygen Uptake Effects of Temperature

Total Volatile Suspended Solids and Active Biomass

Net Biomass Yield and Observed Yield (1) the net biomass yield (2) the observed solids yield Net Biomass Yield Observed Yield

6. MODELING SUSPENDED GROWTH TREATMENT PROCESSES the development of biomass and substrate balances the prediction of effluent biomass and soluble substrate concentrations the prediction of the reactor biomass and MLSS/MLVSS concentrations and amount of waste sludge produced daily (4) the prediction of the oxygen requirements Description of Suspended Growth Treatment Processes Figure 7-12 Schematic diagram of activated-sludge process with model nomenclature: (a) with wasting from the sludge return line and (b)with wasting from the aeration tank.

specific substrate utilization rate If it is assumed that the concentration of microorganisms in the influent can be neglected and that steady-state conditions(dX/dt = 0), average solids retention time (SRT) specific substrate utilization rate

Substrate Mass Balance

Mixed Liquor Solids Concentration and Solids Production the fraction of solids wasted per day and the mixed liquor can be assumed to be a homogeneous mixture of biomass and other solids can be used to calculate the amount of solids wasted for any of the mixed liquor components. For the amount of biomass wasted per day (Px ), the biomass concentration X can be used in place of XT in Eq.

Mixed Liquor Solids Concentration materials balance on the inert material Eq.

Solids Production the amount of VSS produced and wasted daily is determined as follows

The total mass of dry solids wasted per day is based on the TSS, which includes the VSS plus inorganic solids. Inorganic solids are in the influent wastewater (TSS - VSS) and the biomass contains 10 to 15 percent inorganic solids by dry weight. The influent inorganic solids are not soluble, and are assumed captured in the mixed liquor solids and removed in the wasted solids. Equation (7-52) is modified to calculate the solids production in terms of TSS by adding the influent inorganic solids and by calculating the biomass in terms of TSS by assuming a typical biomass VSS/TSS ratio of 0.85. The ratio of VSS/TSS may vary from 0.80 to 0.90.

The Observed Yield S0>>S For wastewaters with no nbVSS in the influent the solids production consists of only active biomass and cell debris, and the observed yield for VSS is as follows For municipal wastewater X0,i/S0 values range from 0.10 to 0.30 g/g with primary treatment and 0.30 to 0.50 without primary treatment. S0>>S

Oxygen Requirements

Design and Operating Parameters Food to Microorganism (F/M) Ratio Specific Substrate Utilization Rate For systems designed for the treatment of municipal wastewater with activated-sludge SRT values in the 20- to 30-d range, the F/M value may range from 0.10 to 0.05 g BOD/g VSSd, respectively. At SRTs in the range of 5 to 7 d, the value may range from 0.3 to 0.5 g BOD/g VSSd, respectively.

Organic Volumetric Loading Rate

Process Performance and Stability The critical SRT value(stabilization does not occur) is called the minimum solids retention residence time SRTmin . Physically, SRTmin is the residence time at which the cells are washed out or wasted from the system faster than they can reproduce. The minimum SRT can be calculated using Eq. , in which S = S0 When washout occurs, the influent concentration S0 is equal to the effluent waste concentration S.

In many situations encountered in waste treatment, S0 is much greater than KS so that can be used to determine the SRT min To ensure adequate waste treatment, biological treatment processes are usually designed and operated with a design SRT value from 2 to 20 times SRTmin. In effect, the ratio of the design SRT (SRTdes) to SRTmin can be considered to be a process safety factor SF against system failure (Lawrence and McCarty,1970).

Modeling Plug-Flow Reactors

7. SUBSTRATE REMOVAL IN ATTACHED GROWTH TREATMENT PROCESSES Typical packing for trickling filters: (a) rock and (b) plastic. diffusion limited The bio-film thickness may range from 100 mm to 10 mm Bio-film VSS concentrations may range from 40 to 100 g/L Uniform growth across the support packing also does not occur, because of periodic sloughing, as well as the hydrodynamics and media configuration

Substrate Flux in Bio-films L = f(fluid properties, velocity)

Substrate Mass Balance for Bio-film The substrate utilization rate within the bio-film at any point Substrate Mass Balance for Bio-film A substrate mass balance around a differential element (dx) For steady-state

Need two boundary conditions * first boundary condition is that the substrate flux at the bio-film surface equals the substrate flux through the stagnant film * second boundary condition is that there is no flux at the packing surface Solutions vary, depending on whether a deep bio-film exists such that the biofilm substrate concentration approaches zero toward the support surface, (2) whether a shallow film exists such that S is a finite value throughout the film, (3) the relative concentration of S compared to K .

Substrate Flux Limitations

8. AEROBIC BIOLOGICAL OXIDATION (1) remove organic constituents and compounds to prevent excessive DO depletion in receiving waters from municipal or industrial point discharges, (2) Remove colloidal and suspended solids to avoid the accumulation of solids and the creation of nuisance conditions in receiving waters, (3) reduce the concentration of pathogenic organisms released to receiving waters Stoichiometry of Aerobic Biological Oxidation Assuming ammonia will serve as the nitrogen source for cell tissue, oxygen is the electron acceptor, and fs for the reaction is 0.59

Growth Kinetics Typical k and Ks values at 20°C Environmental Factors For carbonaceous removal, pH in the range of 6.0 to 9.0 is tolerable, while optimal performance occurs near a neutral pH. A reactor DO concentration of 2.0 mg/L is commonly used, and at concentrations above 0.50 mg/L there is little effect of the DO concentration on the degradation rate.

9. BIOLOGICAL NITRIFICATION Nitrification is the term used to describe the two-step biological process in which Ammonia is oxidized to nitrite and nitrite is oxidized to nitrate The need for nitrification the effect of ammonia on receiving water with respect to DO concentrations and fish toxicity, (2) the need to provide nitrogen removal to control eutrophication, (3) the need to provide nitrogen control for water-reuse applications including groundwater recharge. drinking water maximum contaminant level (MCL) for nitrate nitrogen is 45 mg/L as nitrate or 10 mg/L as nitrogen

Process Description

Stoichiometry of Biological Nitrification Neglecting cell tissue, the amount of alkalinity required to carry out the reaction given in Eq. can be estimated by writing Eq. as follows:

The biomass synthesis reaction

Growth Kinetics : 0.25 - 0.77 g-VSS/g-VSS Effect of DO

BIOLOGICAL DENITRIFICATION

Types of denitrification processes and the reactors used for their implementation substrate driven (preanoxic denitrification) : Modified Ludzak-Ettinger (MLE) process endogenous driven (postanoxic denitrification).

Stoichiometry of Biological Denitrification In biological nitrogen removal processes, the electron donor is typically one of three sources: the bsCOD in the influent wastewater, the bsCOD produced during endogenous decay, an exogenous source such as methanol or acetate. : often used to represent the biodegradable organic matter in wastewater