A comparison of BNR activated sludge systems with membrane and settling tank solid-liquid separation.

Installing membranes for solid-liquid separation into biological nutrient removal (BNR) activated sludge (AS) systems makes a profound difference not only to the design of the membrane bio-reactor (MBR) BNR system itself, but also to the design approach for the whole wastewater treatment plant (WWTP). In multi-zone BNR systems with membranes in the aerobic reactor and fixed volumes for the anaerobic, anoxic and aerobic zones (i.e. fixed volume fractions), the mass fractions can be controlled (within a range) with the inter-reactor recycle ratios. This zone mass fraction flexibility is a significant advantage of MBR BNR systems over BNR systems with secondary settling tanks (SSTs), because it allows changing the mass fractions to optimise biological N and P removal in conformity with influent wastewater characteristics and the effluent N and P concentrations required. For PWWF/ADWF ratios (fq) in the upper range (fq approximately 2.0), aerobic mass fractions in the lower range (f(maer) < 0.60) and high (usually raw) wastewater strengths, the indicated mode of operation of MBR BNR systems is as extended aeration WWTPs (no primary settling and long sludge age). However, the volume reduction compared with equivalent BNR systems with SSTs will not be large (40-60%), but the cost of the membranes can be offset against sludge thickening and stabilisation costs. Moving from a flow unbalanced raw wastewater system to a flow balanced (fq = 1) low (usually settled) wastewater strength system can double the ADWF capacity of the biological reactor, but the design approach of the WWTP changes away from extended aeration to include primary sludge stabilisation. The cost of primary sludge treatment then has to be offset against the savings of the increased WWTP capacity.     

Improvement of nitrogen removal at WWTP Zürich Werdh?elzli after connection of WWTP Zürich-Glatt.

Optimisation of nitrifying activated sludge plants towards nutrient removal (denitrification and enhanced P-removal) leads to a substantial reduction of operating costs and improves effluent and operating conditions. At WWTP Zürich-Werdh?elzli, initially designed for nitrification only, an anoxic zone of 28% of total activated sludge volume was installed and allowed 60% nitrogen elimination besides several other optimisations. In 2001 the operation of WWTP Zürich-Glatt was stopped and the wastewater was connected to WWTP Werdh?elzli. To improve nitrogen removal, WWTP Werdh?elzli co-financed two research projects; one for separate digester supernatant treatment with the anammox process operating two SBRs in series and the other applying NH4 sensors for aeration control in order to decrease energy consumption and raise effluent quality. The results of both projects and the consequences for WWTP Werdh?elzli are discussed in this paper.     

Examples of energy self-sufficient municipal nutrient removal plants

In Austria, two municipal WWTPs (the Strass TP and Wolfgangsee-Ischl TP) operated with nutrient removal and aerobic sludge digestion are now energy self-sufficient. This is the result of a longstanding and on-going optimisation process at both plants including optimal aeration control and control of the aerobic section of the aeration tank to optimise denitrification and prevent degradation of particulate organic matter that should be degraded in the digester. Both TPs are now equipped with energy-efficient CHP units. However, it is maybe more sustainable to use the biogas as biomethane/bio-fuel than in conventional CHP at the WWTP. It is shown that energy self-sufficiency should be in reach at other municipal WWTPs, too.     

Comparative studies on the differently operated trains of the North-Budapest Wastewater Treatment Plant

In order to reduce the pollution load of the Danube, the North-Budapest Wastewater Treatment Plant has been upgraded to enhanced nitrogen removal by establishing a new activated sludge treatment line and modifying the existing unit for nitrification and denitrification. As both the influent flow rate and the influent chemical oxygen demand (COD), biological oxygen demand (BOD(5)) and total suspended solids (TSS) concentration levels remained far below the design values, setting one fourth of the reactor volume out of operation in the Old Line, and operating the nitrification reactor of the New Line with part-time aeration proved to be possible. Analytical data as well as simulation studies supported the advantage of the intermittent-aeration process in efficient N-removal. However, the lengths of the aerated periods have to be increased with decreasing temperature, and thereby effluent total nitrogen (TN) concentration can increase due to decreasing denitrification efficiency. Potential occurrence of low-dissolved oxygen (DO) bulking should be hindered through applying an efficient anoxic selector system.     

Alternate anoxic/aerobic operation for nitrogen removal in a membrane bioreactor for municipal wastewater treatment

A large pilot-scale membrane bioreactor (MBR) with a conventional denitrification/nitrification scheme for municipal wastewater treatment has been run for one year under two different aeration strategies in the oxidation/nitrification compartment. During the first five months air supply was provided according to the dissolved-oxygen set-point and the system run as a conventional predenitrification MBR; then, an intermittent aeration strategy based on effluent ammonia nitrogen was adopted in the aerobic compartment in order to assess the impact on process performances in terms of N and P removal, energy consumption and sludge reduction. The experimental inferences show a significant improvement of the effluent quality as COD and total nitrogen, both due to a better utilization of the denitrification potential which is a function of the available electron donor (biodegradable COD) and electron acceptor (nitric nitrogen); particularly, nitrogen removal increased from 67% to 75%. At the same time, a more effective biological phosphorus removal was observed as a consequence of better selection of denitrifying phosphorus accumulating organisms (dPAO). The longer duration of anoxic phases also reflected in a lower excess sludge production (12% decrease) compared with the standard pre-denitrification operation and in a decrease of energy consumption for oxygen supply (about 50%).     

Biological treatment of sludge digester liquids.

Nitrogen removal in side stream processes offers a good potential for upgrading wastewater treatment plants (WWTPs) that need to meet stricter effluent standards. Removing nutrients from these internal process flows significantly reduces the N-load to the main treatment plant. These internal flows mainly result from the sludge processing and have a high temperature and a high concentration of ammonia. Therefore, the required reactor volumes as well as the required aerobic SRT are small. Generally, biological treatment processes are more economical and preferred over physical-chemical processes. Recently, several biological treatment processes have been introduced for sludge water treatment. These processes are available now on the activated sludge market (e.g. SHARON, ANAMMOX and BABE processes). The technologies differ in concept and in the limitations guiding the application of these processes for upgrading WWTPs. This paper reviews and compares different biological alternatives for nitrogen removal in side streams. The limitations for selecting a technology from the available ones in the activated sludge market are noted and analysed. It is stressed that the choice for a certain process is based on more aspects than pure process engineering arguments.     

Upgrading of Borås wastewater treatment plant based on intelligent process and operation control

Boras wastewater treatment plant is being upgraded to comply with the new effluent standards for nitrogen. Three different configurations have been tested in parallel in full scale in the existing activated sludge plant The Biodenitro operated line showed the best performance because of the flexible adjustment of the nitrification and denitrification capacity. The adjustment was made off-line with the aid of continuous ammonia and nitrate measurements. On-line sensors are now installed permanently at Boris wastewater treatment plant and will be used for on-line dynamic control of the nitrogen removal in a Biodenitro configuration. The plant capacity will be further increased by Killing in the aeration tanks in situations with high hydraulic load.     

Fate of oestrogenic compounds and identification of oestrogenicity in a wastewater treatment process.

Understanding of the fate of oestrogen and oestrogenic compounds is important in improving the removal efficiency for oestrogens in wastewater treatment plants (WWTPs). In this study an attempt was made to clarify the fate of oestrogen, oestrogen sulphates, and oestrogenic compounds (synthetic oestrogen, nonylphenol and its relatives) by an instrumental analysis, and the fate of oestrogenicity by an in vitro assay. The investigation was conducted in an activated sludge WWTP in winter and summer, focusing on identification of the primary substances that induce oestrogenicity. Wastewater samples were analysed by employing the silica-gel fractionation technique in conjunction with two-step column chromatography. The results revealed that, in winter, the WWTP efficiencies for the removal of nitrogen and oestrogens decreased and the oestrone level increased with the progress of the treatment. Oestrone and oestrogenic substances are likely to circulate between the aeration tank and the final sedimentation tank. In summer, however, these compounds were effectively removed in the WWTP. The results of the column chromatography coupled with the bioassay suggested that E1 and E2 are the predominant contributors to the oestrogenicity in the influent, return sludge and effluent of the WWTP. The measurement by the instrumental analysis supported these findings.     

The Polish perspective on adopting EU standards for nitrogen removal at large WWTPs--case studies.

The most challenging issue for existing large WWTPs (>100,000 PE) in Poland will be achievement of the new effluent standards for total nitrogen. Consequently, reliable and accurate information concerning the dimensioning of anoxic compartments is necessary. This study focused on validating to what extent the denitrification rates determined from batch tests were comparable with the rates calculated based on a mass balance over a full-scale activated sludge reactor. The experiments were conducted at two large WWTPs in northern Poland: "Wschod" in Gdansk and "Debogorze" in Gdynia. Two types of batch tests were used to determine the denitrification capability of activated sludge. Lower nitrate utilization rates observed during the full-scale experiments could potentially result from the local disturbances such as nitrate limitation ("Wschod" WWTP) or oxygen penetration to the anoxic zone ("Debogorze" WWTP). These factors should be taken into consideration during the design phase of the anoxic compartments.     

Plant-wide (BSM2) evaluation of reject water treatment with a SHARON-Anammox process.

In wastewater treatment plants (WWTPs) equipped with sludge digestion and dewatering systems, the reject water originating from these facilities contributes significantly to the nitrogen load of the activated sludge tanks, to which it is typically recycled. In this paper, the impact of reject water streams on the performance of a WWTP is assessed in a simulation study, using the Benchmark Simulation Model no. 2 (BSM2), that includes the processes describing sludge treatment and in this way allows for plant-wide evaluation. Comparison of performance of a WWTP without reject water with a WWTP where reject water is recycled to the primary clarifier, i.e. the BSM2 plant, shows that the ammonium load of the influent to the primary clarifier is 28% higher in the case of reject water recycling. This results in violation of the effluent total nitrogen limit. In order to relieve the main wastewater treatment plant, reject water treatment with a combined SHARON-Anammox process seems a promising option. The simulation results indicate that significant improvements of the effluent quality of the main wastewater treatment plant can be realized. An economic evaluation of the different scenarios is performed using an Operating Cost Index (OCI).     

Relevance of the sludge retention time (SRT) as design criteria for wastewater treatment plants for the removal of endocrine disruptors and pharmaceuticals from wastewater.

Wastewater treatment plants (WWTPs) represent a significant source for the input of micro pollutants as endocrine disruptors (EDs) or pharmaceutically active compounds (PhACs) into the aquatic environment. Treatment efficiency of WWTPs often is reported, taking into account only inflow and effluent concentrations without further specification of the WWTP investigated. In order to allow comparison and evaluation of the removal efficiency of different layouts and concepts in wastewater treatment, additional information like the sludge retention time (SRT) and sludge load (F/M ratio) are necessary. Presented results from different WWTPs show correlation of removal of EDs and PhACs to the SRT. Compared to WWTPs with high F/M ratio implementation of the nitrification process on WWTPs results in a significant increase of the removal efficiency for EDs and PhACs. This paper describes an approach to determine comparable removal rates for different activated sludge systems based on mass balance and SRT.     

Reducing the total discharge from a large WWTP by separate treatment of primary effluent overflow.

At many large wastewater treatment plants (WWTPs) the increased hydraulic load, caused by combined sewer systems during storm events, results in primary effluent overflow when the capacity of further treatment is exceeded. Due to stringent effluent standards, regulating the total discharge from the WWTPs, the Rya WWTP in G?teborg and the Sj?lunda WWTP in Malm? will have to reduce the impact of primary effluent overflow. Separate, high rate, precipitation processes operated only during high flow conditions have been investigated in pilot units at the two WWTPs. Precipitation in existing primary settlers operated at a surface loading of 3.75 m/h removed phosphorus to 0.35 mg/l. The Actiflo process was also shown to remove suspended solids and phosphorus well. BOD was reduced by 50-60%. With such processes the overall effluent concentrations from the plants can be reduced significantly. Key upgrading features are small footprints, short start up time and high efficiency.     

Full-scale experience with the deammonification process to treat high strength sludge water -- a case study

More stringent effluent criteria with regard to nitrogen calls for improved nutrient removal techniques in wastewater treatment plants (WWTPs). Besides optimisation of the liquid treatment train of the plants, attention has increasingly centred on the problem of return flows from sludge treatment. One of the most recent developments aimed at the reduction of this nitrogen load is deammonification which has been used at one of Ruhrverband's plants since 2002 by applying a moving bed system. To gain additional experience in operating this process, another full scale plant was modified in 2007 by integration of deammonification, using a SBR system with suspended biomass based on the DEMON(?) control scheme. By using seeding sludge from Strass WWTP in Austria, start-up has been achieved within only 1 day. After stable operation for several months, increasing nitrate concentrations were observed in the effluent of the system indicating growing activity of nitrite oxidising bacteria (NOB). Following severe process deterioration, it was decided to re-start the system again but the same behaviour, i.e. increasing levels of nitrate, was observed once again. Several approaches were used to suppress NOB organisms in full-scale without success, e.g. low oxygen levels and high free ammonia concentrations. Finally, the reduction of the aerobic cycle length during intermittent aeration down to 8 min, followed by an anoxic mixing period of only 18 min was successful in inhibiting the activity of NOB organisms, most probably due to their elevated lag-phase compared with ammonium oxidising bacteria. Today, nitrogen elimination that has been stabilised at more than 80% at a daily volumetric loading rate of 0.5 kg N/(m3 d). The total costs amount to €2.3/kg N(eli).     

Anammox brings WWTP closer to energy autarky due to increased biogas production and reduced aeration energy for N-removal.

Fifty years ago when only BOD was removed at municipal WWTPs primary clarifiers were designed with 2-3 hours hydraulic retention time (HRT). This changed with the introduction of nitrogen removal in activated sludge treatment that needed more BOD for denitrification. The HRT of primary clarification was reduced to less than one hour for dry weather flow with the consequence that secondary sludge had to be separately thickened and biogas production was reduced. Only recently the ammonia rich digester liquid (15-20% of the inlet ammonia load) could be treated with the very economic autotrophic nitritation/anammox process requiring half of the aeration energy and no organic carbon source compared to nitrification and heterotrophic denitrification. With the introduction of this new innovative digester liquid treatment the situation reverts, allowing us to increase HRT of the primary clarifier to improve biogas production and reduce aeration energy for BOD removal and nitrification at similar overall N-removal.     

Removal efficiency of pharmaceuticals and personal care products with varying wastewater treatment processes and operating conditions - conception of a database and first results.

We created a database in order to quantitatively assess the occurrence and removal efficiency of PPCPs in WWTPs. From 113 scientific publications, we compiled 5887 data on the concentrations and loads of PPCPs in WWTP influents and effluents, and on their removal efficiency. The first outputs of our database include: (1) a list of the most frequently studied molecules, their frequency of detection, their mean concentration and removal in liquid influent and effluent; (2) a comparison of the removal efficiency for different WWTP processes; (3) a study of the influence of the operating conditions (sludge and hydraulic retention times).     

Occurrence and reduction of pharmaceuticals in the water phase at Swedish wastewater treatment plants

During the last decade, several screening programs for pharmaceuticals at Swedish wastewater treatment plants (WWTPs) have been conducted by research institutes, county councils, and wastewater treatment companies. In this study, influent and effluent concentrations compiled from these screening programs were used to assess the occurrence and reduction of non-antibiotic pharmaceuticals for human usage. The study is limited to full-scale WWTPs with biological treatment. Based on the data compiled, a total of 70 non-antibiotic pharmaceuticals have been detected, at concentrations ranging from a few ng/L to several μg/L, in the influent water. The influent concentrations were compared with the sale volumes and for many pharmaceuticals it was shown that only a small fraction of the amount sold reaches WWTPs as dissolved parent compounds. Pharmaceuticals with low reduction degrees at traditional WWTPs were identified. Further comparison based on the biological treatment showed lower reduction degrees for several pharmaceuticals in trickling filter plants compared with activated sludge plants with nitrogen removal.     

Solved upscaling problems for implementing deammonification of rejection water.

So far, extremely efficient metabolic pathways for nitrogen removal exclusively by autotrophic organisms are well established in scientific literature but not in practice. This paper presents results from the successful implementation of rejection water deammonification in a full-scale single sludge system at the WWTP Strass, Austria. Anaerobic ammonia oxidising biomass has been accumulated during a 2.5 year start-up period when the reactor size was gradually scaled up in the steps. The pH-controlled deammonification system (DEMON) has reached a design capacity of eliminating approximately 300 kg of nitrogen per day. Energy savings outperform expectations, decreasing the mean specific demand for compressed air from 109 m3(kg N)(-1) to 29 m3(kg N)(-1). Dominance of autotrophic metabolism is confirmed by organic effluent loads topping influent loads.     

Optimisation of storage driven denitrification by using on-line specific oxygen uptake rate monitoring during SND in a SBR.

This study builds on previous experience of maximising the formation of COD as poly-hydroxybutyrate (PHB) and now describes a feedback technique of preserving the use of PHB for denitrification resulting in enhanced nitrogen removal rather than allowing its wasteful oxidation by oxygen. The feedback technique uses on-line SOUR monitoring for detecting the end-point of nitrification and controlling the aerobic phase length accordingly. The laboratory SBR was operated such that all organic substrate (acetate) was rapidly converted to PHB, which then served as the electron donor for nitrogen removal via simultaneous nitrification and denitrification (SND) during the aerobic phase (up to 70% SND). During SBR cycling with a fixed aeration length (240 minutes), PHB was unnecessarily oxidised after ammonium depletion, resulting in little denitrification and poor total nitrogen removal (69%). However, when the aerobic phase length was controlled via the SOUR, up to 1.8 CmM PHB (58 mg L(-1) COD) could be preserved, enabling improved total nitrogen removal (86%). The drop in the SOUR after ammonium depletion was a reproducible event that could be detected even when using raw wastewater and fresh activated sludge. The SOUR-control technique holds promise to build up PHB over a number of SBR cycles. While advanced oxygen-control is used for improved N-removal in several existing WWTPs, this study investigates the importance of oxygen control with relevance to PHB driven SND in sequencing batch reactors.     

Upgrading for nitrogen removal under severe site restrictions

The Rya WWTP in Göteborg, Sweden is a highly loaded activated sludge plant which has been upgraded to remove nitrogen and to increase the biologically treated flow by 33% to reduce bypassing. Severe site restrictions made it difficult to increase the plant area. This was solved using a compact process based on tertiary nitrification in trickling filters and recirculation to a highly loaded activated sludge unit for denitrification. The necessary volumes were achieved by expanding the plant upwards, thus making it possible to place the trickling filters a in part of the area occupied by the former aeration basins. The recirculation of trickling filter effluent made it necessary to double the secondary settler capacity. This was solved by retrofitting the existing settlers as stacked settlers with a second tray on top of the original basin. The considerations behind process selection and plant design and costs are described.     

Nitrogen removal characteristics analyzed with gas and microbial community in thermophilic aerobic digestion for piggery waste treatment.

In order to characterize the nitrogen conversion characteristics in a thermophilic aerobic digestion (TAD) system, a laboratory study has been conducted with the analysis of effluent gas and microbial community in the sludge samples. The lab TAD system was operated with HRT of 3 days and 60 degrees C. Based on the nitrogen mass balance, it has been found that about 2/3 of the daily load of nitrogen was converted to the gaseous form of nitrogen whereas cellular transformation and unmetabolized nitrogen accounted for about 1/3. Among the gaseous nitrogen transformation, significant amount of influent nitrogen had been converted to N2 gas (29% of influent N) and N2O (9% of influent N). Ammonia conversion was only 28% of influent N. The detection of N2O gas is a clear indication of the biological nitrogen reduction process in the thermophilic aerobic digester. No conclusive evidence for the existence of aerobic deammonification has been found. The microbial community analysis showed that thermophilic bacteria such as Bacillus thermocloacae, Bacillus sp. and Clostridial groups dominated in this TAD reactor. The diverse microbial community in TAD sludge may play an important role in removing both strong organics and nitrogen from piggery waste.