Peter Dold

Denitrification behaviour in biological excess phosphorus removal activated sludge systems

Abstract A literature review of denitrification behaviour in biological excess phosphorus removal (BEPR) activated sludge systems was performed. Wentzel et al. (1989a,b) ( Wat. S.A. , 15, 71–88; Wat. S.A. , 15, 89–102) excluded denitrification by poly P organisms in modelling the BEPR process as observations indicated that minimal denitrification ocurred in laboratory systems comprised mainly of poly P organisms. In contrast, results of microbiological studies and many continuous and batch reactor experimental studies reviewed here indicate that a significant fraction of the poly P organisms can use nitrate as an electron acceptor in the absence of oxygen for oxidation of stored PHB and simultaneous uptake of phosphorus. Quantifying the extent of denitrification by poly P organisms in a particular system requires further information on factors determining the fraction of organisms capable of denitrification and the stoichiometry of the process.

COD and nitrogen mass balances in activated sludge systems

Abstract COD and nitrogen balances were performed on four different types of laboratory-scale activated sludge system: aerobic, anoxic, anoxic-aerobic and anaerobic-anoxic-aerobic (biological excess phosphorus removal systems). The systems included a variety of configurations, with differing wastewater characteristics and operating parameters. The results suggest that good COD balances are to be expected in aerobic and anoxic-aerobic systems. Systems incorporating anaerobic zones exhibit low COD balances (less than 80%). Fermentation in the anaerobic zone apparently is implicated in this “loss” of COD. The consequences of the COD “loss” include both a significant decrease in oxygen requirements and in sludge production compared to aerobic or anoxic-aerobic systems. Possible mechanisms for the loss of COD and areas which require further study are discussed.

Biodegradation of the endogenous residue of activated sludge.

This study evaluated the potential biodegradability of the endogenous residue in activated sludge subjected to batch digestion under either non-aerated or alternating aerated and non-aerated conditions. Mixed liquor for the tests was generated in a 200 L pilot-scale aerobic membrane bioreactor (MBR) operated at a 5.2 days SRT. The MBR system was fed a soluble and completely biodegradable synthetic influent composed of sodium acetate as the sole carbon source. This influent, which contained no influent unbiodegradable organic or inorganic materials, allowed to generate sludge composed of essentially two fractions: a heterotrophic biomass X(H) and an endogenous residue X(E), the nitrifying biomass being negligible (less than 2%). The endogenous decay rate and the active biomass fraction of the MBR sludge were determined in 21-day aerobic digestion batch tests by monitoring the VSS and OUR responses. Fractions of X(H) and X(E): 68% and 32% were obtained, respectively, at a 5.2 days SRT. To assess the biodegradability of X(E), two batch digestion units operated at 35 degrees C were run for 90 days using thickened sludge from the MBR system. In the first unit, anaerobic conditions were maintained while in the second unit, alternating aerated and non-aerated conditions were applied. Data for both units showed apparent partial biodegradation of the endogenous residue. Modeling the batch tests indicated endogenous residue decay rates of 0.005 d(-1) and 0.012 d(-1) for the anaerobic unit and the alternating aerated and non-aerated conditions, respectively.

The use of crossflow microfiltration to enhance the performance of an activated sludge reactor

Crossflow microfiltration (CFMF) was investigated as a substitute for secondary settling in the activated sludge process, to retain a greater biomass concentration in the reactor and increase the reactor chemical oxygen demand (COD) degrading capacity. The system was operated at six different steady-state conditions. Sludge age was varied between 5 and 20 days and chemical oxygen demand (COD) loadings ranged between 7.5 and 45.0 g l−1. The reactor suspended solids concentration was increased from 0.95 to 19.47 g l−1, over three times the secondary settling maximum of 6.0 g l−1. Effluent quality was good with 0.013 g l−1 suspended solids and COD removal above 97%. With secondary settling the effluent suspended solids concentration is commonly 0.020 g l−1 although this figure increases as the maximum reactor solids concentration exceeds 6.0 g l−1. For CFMF it was necessary to pump sludge under pressure through the filtration system. The pumping resulted in a particle size of 2 μm, in comparison with floc sizes of 100 μm in activated sludge systems operating with secondary settling. The size reduction had no deleterious effect on the reactor COD removal capacity. It was noticed, however, that it was not possible to settle the sludge particles.

The enhancement of upflow anaerobic sludge bed reactor performance using crossflow microfiltration

Abstract Crossflow microfiltration (CFMF) was used to filter liquor removed from the top of a laboratory-scale upflow anaerobic sludge-bed (UASB) reactor. Prior to filtration the COD removal was 91.7 %, with 1.0 g l−1 of suspended solids at the top of the reactor. During 17 days of operation with filtration the suspended solids concentration was increased to 5.9 g l−1. The maximum solids concentration at which the reactor could be operated was not reached. The effluent quality was high (0.050 g l−1 suspended solids). The COD removal was increased to between 98 and 99%. It is possible that returning liquor aerated the top zone of the reactor, promoting the growth of aerobic organisms on residual COD material. Alternatively the increased capacity could have been due to the biomass retention. The coupling of the UASB reactor to the CFMF unit led to an improvement in the effluent from the system, i.e. lower effluent suspended solids concentration and increased COD removal capacity due to aerobic polishing

Characterizing denitrification kinetics at cold temperature using various carbon sources in lab-scale sequencing batch reactors.

Wastewater treatment plants in the Chesapeake Bay region are becoming more interested in external carbon sources for denitrification. This is in response to the recent regulations to remediate the Chesapeake Bay, which will limit effluent total nitrogen to near 3 mg/L for plants, thus requiring near complete elimination of inorganic nitrogen species. Since sufficient internal carbon is usually not available for complete denitrification, external carbon is needed to supplement internal sources. Of particular interest is the use of an alternate external carbon source to replace the least expensive source methanol. This study focuses on three commonly available external carbon sources: methanol, ethanol and acetate. The aim of this study was to obtain the specific denitrification rate (SDNR) of the substrates under several conditions. Sequencing batch reactors (SBRs) were set up to first grow biomass to the specified substrate while in situ SDNRs were conducted concurrently. Once the biomass was grown with the corresponding substrate, a series of ex situ SDNRs were performed using various biomass/substrate combinations to evaluate response to substrate combinations at 13 degrees C. Results from this study indicate that the SDNRs for biomass grown on methanol, ethanol and acetate were 9.2 mg NO(3)-N/g VSS/hr, 30.4 mg NO(3)-N/gVSS/hr and 31.7 mg NO(3)-N/g VSS/hr, respectively, suggesting that acetate and ethanol were equally effective external carbon sources followed by much lower SDNR using methanol. Ethanol could be used with methanol biomass with similar rates as that of methanol. Additionally, methanol was rapidly acclimated to ethanol grown biomass suggesting that the two substrates could be interchanged to grow respective populations with a minimum lag period.

Biodegradation of the endogenous residue of activated sludge in a membrane bioreactor with continuous or on-off aeration.

The goal of this study was to determine the effect of a long sludge retention time on the biodegradation of the endogenous residue in membrane digestion units receiving a daily feed of sludge and operated under either aerobic or intermittently aerated (22 h off-2 h on) conditions. The mixed liquor for these experiments was generated in a 10.4 day sludge retention time membrane bioreactor fed with a synthetic and completely biodegradable influent with acetate as the sole carbon source. It had uniform characteristics and consisted of only two components, heterotrophic biomass X(H) and endogenous residue X(E). Membrane digestion unit experiments were conducted for 80 days without any sludge wastage except for some sampling. The dynamic behaviour of generation and consumption of filtered organic digestion products was characterized in the membrane digestion unit systems using three pore filter sizes. Results from this investigation indicated that the colloidal matter with size between 0.04 μm and 0.45 μm was shown to contain a recalcitrant fraction possibly composed of polysaccharides bound to proteins which accumulated in the membrane digestion unit under both conditions. Modelling the membrane digestion unit results by considering a first-order decay of the endogenous residue allowed to determine values of the endogenous residue decay rate of 0.0065 and 0.0072 d(-1) under fully aerobic and intermittently aerated conditions, respectively. The effect of temperature on the endogenous decay rate was assessed for the intermittently aerated conditions in batch tests using thickened sludge from tests gave an endogenous decay rate constant of 0.0075 d(-1) at 20 °C and an Arrhenius temperature correction factor of 1.033.

Modelling the degradation of endogenous residue and ‘unbiodegradable’ influent organic suspended solids to predict sludge production

Activated sludge models have assumed that a portion of organic solids in municipal wastewater influent is unbiodegradable. Also, it is assumed that solids from biomass decay cannot be degraded further. The paper evaluates these assumptions based on data from systems operating at higher than typical sludge retention times (SRTs), including membrane bioreactor systems with total solids retention (no intentional sludge wastage). Data from over 30 references and with SRTs of up to 400 d were analysed. A modified model that considers the possible degradation of the two components is proposed. First order degradation rates of approximately 0.007 d(-1) for both components appear to improve sludge production estimates. Factors possibly influencing these degradation rates such as wastewater characteristics and bioavailability are discussed.

Characterization of the heterotrophic biomass and the endogenous residue of activated sludge

The activated sludge process generates an endogenous residue (X(E)) as a result of heterotrophic biomass decay (X(H)). A literature review yielded limited information on the differences between X(E) and X(H) in terms of chemical composition and content of extracellular polymeric substances (EPS). The objective of this project was to characterize the chemical composition (x, y, z, a, b and c in C(x)H(y)O(z)N(a)P(b)S(c)) of the endogenous and the active fractions and EPS of activated sludge from well designed experiments. To isolate X(H) and X(E) in this study, activated sludge was generated in a 200L pilot-scale aerobic membrane bioreactor (MBR) fed with a soluble and completely biodegradable synthetic influent of sodium acetate as the sole carbon source. This influent, which contained no influent unbiodegradable organic or inorganic particulate matter, allowed the generation of a sludge composed essentially of two fractions: heterotrophic biomass X(H) and an endogenous residue X(E), the nitrifying biomass being negligible. The endogenous decay rate and the active biomass fraction of the MBR sludge were determined in 21-day aerobic digestion batch tests by monitoring the VSS and OUR responses. Fractions of X(H) and X(E) were respectively 68% and 32% in run 1 (MBR at 5.2 day SRT) and 59% and 41% in run 2 (MBR at 10.4 day SRT). The endogenous residue was isolated by subjecting the MBR sludge to prolonged aerobic batch digestion for 3 weeks, and was characterized in terms of (a) elemental analysis for carbon, nitrogen, phosphorus and sulphur; and (b) content of EPS. The MBR sludge was characterized using the same procedures (a and b). Knowing the proportions of X(H) and X(E) in this sludge, it was possible to characterize X(H) by back calculation. Results from this investigation showed that the endogenous residue had a chemical composition different from that of the active biomass with a lower content of inorganic matter (1:4.2), of nitrogen (1:2.9), of phosphorus (1:5.3) and of sulphur (1:3.2) but a similar content of carbon (1:0.98). Based on these elemental analyses, chemical composition formulae for X(H) and X(E) were determined as CH(1.240)O(0.375)N(0.200)P(0.0172)S(0.0070) and CH(1.248)O(0.492)N(0.068)P(0.0032)S(0.0016), respectively. Data from EPS analyses also confirmed this difference in structure between X(E) and X(H) with an EPS content of 11-17% in X(E)versus 26-40% in X(H).

Biological Nutrient Removal in Municipal Wastewater Treatment: New Directions in Sustainability

To control eutrophication in receiving water bodies, biological nutrient removal (BNR) of nitrogen and phosphorus has been widely used in wastewater treatment practice, both for the upgrade of existing wastewater treatment facilities and the design of new facilities. However, implementation of BNR activated sludge AS systems presents challenges attributable to the technical complexity of balancing influent chemical oxygen demand (COD) for both biological phosphorus (P) and nitrogen (N) removal. Sludge age and aerated/unaerated mass fractions are identified as key parameters for process optimization. Other key features of selected BNR process configurations are discussed. Emerging concerns about process sustainability and the reduction of carbon footprint are introducing additional challenges in that influent COD, N, and P are increasingly being seen as resources that should be recovered, not simply removed. Energy recovery through sludge digestion is one way of recovering energy from influent wastewater b...