REVIEW - Recent developments in biological nutrient removal

Biological nitrogen (N) and phosphorus (P) removal from municipal wastewater with the activated sludge (AS) system has been the preferred technology for the last 40 years. While several questions remain to be answered for more consistent, reliable and stable performance for enhanced biological P removal (EBPR), recent developments in this technology have focused on (i) increasing capacity and reducing the plant space footprint and (ii) improving N removal. To increase capacity and reduce AS system space, (a) integrated fixed-film activated sludge (IFAS), (b) external nitrification, (c) membrane, (d) aerobic granulation BNR systems and (e) more efficient N removal bioprocesses (anammox and nitrite shunt) have been developed. With IFAS, fixed media are added to the aerobic activated sludge reactor to make nitrification independent of the suspended AS sludge age. With external nitrification, nitrification is achieved in a side-stream fixed media reactor, which removes the size-defining nitrification process from the suspended AS system and halves its sludge age, improves sludge settleability and increases capacity. With membranes, secondary settling tanks are replaced with in-reactor membranes for solid-liquid separation. With aerobic granulation, the activated sludge process is controlled to form fast-settling granules comprising heterotrophs, nitrifiers, denitrifiers and phosphorus-accumulating organisms (PAOs) in a sequencing batch (SBR) type reactor – the granules not only settle fast but the SBR-type operation also removes the need for secondary settling tanks allowing higher reactor solids concentrations and hence smaller reactors. To achieve N removal more efficiently, methods are being developed to (i) short-circuit nitrification-denitrification (ND) by preventing nitrate formation and enforcing ND over nitrite – this requires less oxygen and organics than ND over nitrate allowing lower N concentrations to be achieved for the same influent organics concentration and oxygen supply, and (ii) encouraging the growth of anammox bacteria in the activated sludge which remove N autotrophically by combining ammonia and nitrite to form nitrogen gas – this halves oxygen demand for nitrification and requires no organics. These recent developments in BNR technology are briefly reviewed in this paper

Biological sulphate reduction with primary sewage sludge in an upflow anaerobic sludge bed (UASB) reactor – Part 3: Performance at 20°C and 35°C

The performance of 2 biological sulphate reduction (BSR) upflow anaerobic sludge bed (UASB) reactors fed primary sewage sludge (PSS) and sulphate, one at 20oC (R2) and one at 35oC (R1) is described. To maintain the effluent sulphate concentration below 250 mgSO42-/., the hydraulic retention time (HRT) and bed solids retention time (SRT or sludge age) both needed to be longer and the feed primary sewage sludge (PSS) COD to SO4 2- ratio higher at 20oC than at 35oC, viz. 20.4 to 21.0 h, 24 d and 1.75 gCOD/gSO4 2- at 20oC and 16.4 to 17.0 h, 21 d and 1.75 gCOD/gSO4 2- at 35oC respectively. The longer HRT, SRT and higher feed PSS COD/ SO4 2- ratio is a consequence of a slower PSS hydrolysis/acidogenesis rate at 20oCresulting in a lower biodegradable particulate organics conversion to volatile fatty acids (VFA). Solid liquid separation in both systems was good yielding average particulate and soluble organic COD concentrations of (150 and 100 mgCOD/. for R1; 138 and 96 mgCOD/. for R2). The sulphate reduction was >90% in both systems. The UASB reactor R1 (at 35oC) was also operated at an increased influent sulphate concentration (1 800 mgSO4 2-/.) to investigate the inhibition effect by un-dissociated hydrogen sulphide generated from the reduction of this high sulphate concentration. It was found that a highsulphate reduction (~ 92%) was maintained even at the relatively low HRT of 18.5 h. The COD and S mass balances above 95% were achieved over both systems indicating that the performance data obtained from them is reliable for developing and calibrating mathematical models

The change of pH under a paint film due to cathodic protection

The diffusion of OH−-ions and H2 formed during cathodic protection through a paint film is studied. The diffusion equation is solved for non-stationary conditions and from this the steady state is also derived. It is shown that under usual operating conditions of cathodic protection of ships the stationary state is reached in about 15 days. The danger of paint deterioration through high alkalinity and blistering is found to increase with current density and paint film thickness and with decreasing diffusion coefficient in the paint. For practical calculations a better knowledge of diffusion coefficients in paints is needed

Biological sulphate reduction with primary sewage sludge in an upflow anaerobic sludge bed reactor – Part 6: Development of a kinetic model for BSR

A 2-phase (aqueous-gas) kinetic model for biological sulphate reduction (BSR) using primary sewage sludge (PSS) as carbon source is presented. The methanogenic anaerobic digestion (AD) model of Sötemann et al. (2005) is extended by adding the biological, chemical and physical processes associated with BSR, i.e. propionic acid degrading sulphate-reducing bacteria (SRB), acetoclastic SRB and hydrogenotrophic SRB, the aqueous weak acid/base chemistry processes of the sulphate and sulphide systems and an aqueous-gas sulphide exchange process. The model is validated with experimental data from 2 upflow anaerobic sludge bed (UASB) reactors fed various PSS COD/SO42- ratios under constant flow and load conditions at 35°C and 20°C. The kinetic model results, including the reactor pH (within 0.1 pH unit) compare well with the experimental results and with those calculated from a steady-state BSR model. The kinetic model confirms that: (1) at ambient temperature (20°C), the hydrolysis rate is significantly reduced compared with that at 35°C, which requires a longer sludge age (larger bed volume) in the UASB reactor; (2) the hydrolysis rate of the PSS biodegradable particulate organics (BPO) is the same under methanogenic and sulphidogenic conditions; (3) the PSS BPO are carbon deficient for BSR in that more electrons are donated than carbon supplied for the required alkalinity increase, with the result that the sulphide system supplies the alkalinity deficit; and (4) due to (3) there is zero CO2 gas generation and in effect the sulphide system establishes the reactor pH. This observation allows the carbon content of the utilised organics to be determined from the H2CO3* alkalinity increase in the reactor, which can be simply measured by titration methods. Keywords: biological sulphate reduction, primary sewage sludge, upflow anaerobic sludge bed reactor, dynamic model, kinetics, stoichiometry, mixed weak acid/base chemistr

Biological sulphate reduction with primary sewage sludge in an upflow anaerobic sludge bed reactor – Part 5: Steady-state model

This paper describes the development of a steady-state anaerobic digestion model for biological sulphate reduction using primary sewage sludge (PSS) as substrate. The model comprises: a chemical oxygen demand (COD) based hydrolysis kinetics part in which the PSS biodegradable COD and sulphate removals are calculated for given hydraulic and sludge retention times; a C, H, O, N, P, S, COD and charge mass balance stoichiometry part in which the alkalinity generated (from both the HCO3- and HS-) is determined from the COD and sulphate removals; and an inorganic carbon (CO2) and sulphide mixed weak acid/base chemistry part in which the digester pH is calculated from the HCO3- and HS- species formed. From the stoichiometry, it was found that the PSS is carbon limited in that it does not generate sufficient HCO3- alkalinity for the sulphate reduction, i.e., its COD/C ratio is too high which accounts for the observed zero gas (CO2) generation. The H2S/HS- system provides the alkalinity shortfall and establishes the system pH. Once developed and calibrated, the model results were compared with experimental data from 2 laboratory-scale upflow anaerobic sludge bed reactors (operated at 35oC and 20°C respectively) fed PSS and sulphate. The predicted COD and sulphate removals, alkalinity and digester pH correspond very well to the measured data. The model assists in identifying design and operation parameters sensitive to the system and provides a basis for developing an integrated biological, chemical and physical process dynamic model. Keywords: biological sulphate reduction, primary sewage sludge, upflow anaerobic sludge bed reactor, steady state model, kinetics, stoichiometry, mixed weak acid/base chemistr

Mass balance-based plant-wide wastewater treatment plant models – Part 1: Biodegradability of wastewater organics under anaerobic conditions

From an experimental and theoretical investigation of the continuity of wastewater organic chemical oxygen demand (COD) and nitrogen (N) compounds along the link connecting the primary settling tank (PST) and anaerobic digester (AD), it was found that the primary sludge (PS) characteristics, viz. the biodegradable and unbiodegradable soluble and particulate COD and N component concentrations, need to be calculated from mass balances around the PST so that the organic and N concentrations conform to continuity principles, and the influent unbiodegradable particulate organics determined from response of the activated sludge (AS) system are also unbiodegradable under AD conditions. Water SA Vol.32 (3) 2006: pp.269-27

Modelling organic material in activated sludge systems

A simple predictive model for the activated sludge reactor inorganic suspended solids (ISS) concentration is presented. It is based on the accumulation of influent ISS in the reactor and an ordinary heterotrophic organism (OHO) ISS content (fiOHO) of 0.15 mg ISS/mgOHOVSS and a variable phosphate accumulating organism (PAO) ISS content (fiPAO) proportional to their P content (fXBGP). The model is validated with data from 21 investigations conducted over the past 15 years on 30 aerobic and anoxic-aerobic nitrification denitrification (ND) systems and 18 anaerobic-anoxic-aerobic ND biological excess P removal (BEPR) systems variously fed artificial and real wastewater and operated from 3 to 20 d sludge age. The predicted reactor VSS/TSS ratio reflects the observed relative sensitivity to sludge age, which is low, and to BEPR, which is high. For effective use of the model for design, two significant issues require attention: measurement of the influent ISS concentration, which is not commonly done in wastewater characterisation analyses; and estimating a priori the P content of PAOs (fXBGP), which can vary considerably depending on the extent of anoxic P uptake BEPR that takes place in the system. Some guidance on selection of the mixed liquor VSS/TSS ratio for design is given

Greenhouse gases from membrane bioreactors: Mathematical modelling, sensitivity and uncertainty analysis

In this study a new mathematical model to quantify greenhouse gas emissions (namely, carbon dioxide and nitrous oxide) from membrane bioreactors (MBRs) is presented. The model has been adopted to predict the key processes of a pilot plant with pre-denitrification MBR scheme, filled with domestic and saline wastewater. The model was calibrated by adopting an advanced protocol based on an extensive dataset. In terms of nitrous oxide, the results show that an important role is played by the half saturation coefficients related to nitrogen removal processes and the model factors affecting the oxygen transfer rate in the aerobic and MBR tanks. Uncertainty analysis showed that for the gaseous model outputs 88\ue2\u80\u9393% of the measured data lays inside the confidence bands showing an accurate model prediction

Application of the activated sludge model to aerated lagoons

The different kinds of aerated lagoons, which exclude anaerobic pre-treatment ponds, are described and the design approach for aerated lagoons is explained. This hinges around ensuring that the 1st lagoon is suspension mixed and the second and any additional are facultative. Selection of the retention time for the 1st lagoon is important to ensure complete utilization of the influent biodegradable organics. Minimum retention times to achieve this at 14°C and 22°C were determined with the general activated sludge kinetic simulation model for (i) readily biodegradable soluble organics (BSO) only, (ii) slowly biodegradable particulate organics (BPO) only, (iii) real municipal wastewater (20% BSO and 80% BPO) and (iv) real municipal wastewater with 5% OHO active VSS mass seed. The minimum hydraulic retention times for these four cases are: at 14°C 1.3, 3.0, 2.0 and 1.5 d, respectively, and at 22°C 0.3, 2.0, 1.2 and 1.0 d, respectively. From a comparison of the simulation results with the steady-state model calculations, washout of OHOs takes place at about 75% of these retention times. Approximate equations to estimate the power requirements for aeration by mechanical surface aerators and mixing are given. These equations are combined with those of the steady-state activated sludge lagoon model for calculating the oxygen requirements and the aeration power density (W/m3) in each lagoon. With these equations, it is shown that influent COD concentration needs to be between an upper and lower limit band to ensure that the 1st lagoon is suspension mixed and the second lagoon is facultative. This COD concentration band decreases as the influent flow increases. The important conclusion arising from this is that if the aerated lagoon system is applied for small rural communities, where land for these large systems is likely to be available, then additional mixing energy above that for aeration will need to be provided to ensure that the 1st lagoon is suspension mixed – this additional aeration cost makes it unlikely that aerated lagoons will be applied for municipal wastewater treatment. Matching mixing and aeration power requirements for industrial organic wastewaters is easier because these usually are significantly stronger than municipal wastewaters

The removal of N and P in aerobic and anoxic-aerobic digestion of waste activated sludge from biological nutrient removal systems

Biological nutrient removal (BNR) activated sludge (AS) systems produce a waste activated sludge (WAS) that is rich in nitrogen (N) and phosphorus (P). When this sludge is thickened to 3–6% total suspended solids (TSS) and digested (aerobic or anaerobic), a high proportion of N and P are released to the bulk liquid resulting in high concentrations of ammonia/nitrate and orthophosphate up to several hundred mg/ℓ (without denitrification or P precipitation). This research investigates P removal by P precipitation in anoxic-aerobic digestion of P-rich BNR system WAS. The experimental setup for this work was a lab-scale membrane UCT BNR system fed real settled sewage with added acetate, orthophosphate, and cations Mg and K to increase biological excess P removal. This WAS was fed to batch aerobic digesters at various TSS concentrations, and to two 20-day retention time continuous anoxic-aerobic digesters (AnAerDig) with aeration cycles of 3-h air on and 3-h air off, one fed concentrated WAS (20 g TSS/ℓ ) and the other fed diluted WAS (3 g TSS/ℓ). Nitrogen removal has been discussed in the previous paper. This paper focuses on the P removal by P precipitation observed in the batch tests and continuous systems. The rate of polyphosphate release (bGP) during batch aerobic digestion at low TSS without P precipitation was found to be 2.5 times faster than the endogenous respiration rate (bG) of phosphorus accumulating organics (PAO), i.e. bGP = 0.1/d. This rate was then applied to the high-TSS aerobic batch tests and continuous anoxic-aerobic digesters to estimate the P precipitation at various TSS concentrations, with and without additional Mg or Ca dosing. Newberyite (MgHPO4·3H2O) and amorphous tricalcium phosphate (ACP or TCP, Ca3(PO4)2·xH2O) are found to be the most common phosphate precipitates.Keywords: biological excess phosphorus removal, waste activated sludge, anoxic-aerobic digestion, phosphaterelease, mineral precipitatio