K. Svardal

Biogas from sugar beet press pulp as substitute of fossil fuel in sugar beet factories.

Sugar beet press pulp (SBP) accumulates as a by-product in sugar factories and it is generally silaged or dried to be used as animal food. Rising energy prices and the opening of the European Union sugar market has put pressure on the manufacturers to find alternatives for energy supply. The aim of this project was to develop a technology in the treatment of SBP that would lead to savings in energy consumption and would provide a more competitive sugar production from sugar beets. These goals were met by the anaerobic digestion of SBP for biogas production. Lab-scale experiments confirmed the suitability of SBP as substrate for anaerobic bacteria. Pilot-scale experiments focused on process optimization and procedures for a quick start up and operational control. Both single-stage and two-stage process configurations showed similar removal efficiency. A stable biogas production could be achieved in single-stage at a maximum volumetric loading rate of 10 kgCSB/(m(3) x d). Degradation efficiency was 75% for VS and 72% for COD. Average specific gas production reached 530 NL/kgCOD(SBP) or 610 NL/kgVS(SBP). (CH(4): 50 to 53%). The first large-scale biogas plant was put into operation during the sugar processing period 2007 at a Hungarian sugar factory. Digesting approximately 50% of the SBP (800 t/d, 22%TS), the biogas produced could substitute about 40% of the natural gas required for the thermal energy supply within the sugar processing.

Aeration of anaerobically digested sewage sludge for COD and nitrogen removal: optimization at large-scale

The paper will report about the experiences at an Austrian large wastewater treatment plant of 720,000 population equivalents, where anaerobically digested sewage sludge is further stabilised under aerobic conditions. Enhanced stabilisation of the anaerobically digested sludge was required at the plant in order to get a permit for landfill disposal of the dewatered stabilized sludge. By implementing a post-aeration treatment (SRT approximately 6d; 36 degrees C) after anaerobic digestion the organic content of the anaerobically well digested sludge can be decreased by 16%. Investigations on site showed that during digested sludge post-aeration anoxic phases for denitrification are needed to provide stable process conditions. In this way the pH value can be kept in a more favourable range for micro-organisms and concrete structures. Additionally, inhibition of the biological process due to nitrite accumulation can be avoided. By optimising the aeration/pause ratio approximately 45% of total nitrogen in digested sludge can be removed. This significantly improves nitrogen removal efficiency at the wastewater treatment plant. NH(4)-removal occurs mainly through nitritation and denitritation with an efficiency of 98%. The costs/benefit analysis shows that post-aeration of digested sludge results in an increase of total annual costs for wastewater treatment of only 0.84%, corresponding to 0.19 Euro/pe/a. Result of molecular biological analyses (DGGE) indicate that all four ammonium-oxidizing bacteria species present in activated sludge can survive anaerobic digestion, but only two of them can adapt in the digested sludge post-aeration tanks. Additionally, in the post-aerated digested sludge a further ammonium-oxidizing bacteria species was identified.

Post-aeration of anaerobically digested sewage sludge for advanced COD and nitrogen removal: results and cost-benefit analysis at large-scale.

At a large Austrian municipal wastewater treatment plant enhanced stabilisation of anaerobically digested sewage sludge was required in order to get a permit for landfill disposal of the dewatered stabilized sludge. By implementing a post-aeration treatment after anaerobic digestion the organic content of the anaerobically well digested sludge can be decreased by 16%. Investigations at this plant showed that during digested sludge post-aeration anoxic phases are needed to provide stable process conditions. In this way the pH value can be kept in a more favourable range for micro-organisms and concrete structures. Additionally, under the process conditions applied nitrite accumulation would inhibit the stabilisation process if denitrification is not adequately applied. By optimising the aeration/pause ratio approximately 45% of total nitrogen in digested sludge can be removed. NH4-removal occurs through nitrification and denitrification with an efficiency of 98%. This significantly improves nitrogen removal efficiency at the wastewater treatment plant. The costs/benefit analysis shows that post-aeration of digested sludge results in an increase of total annual costs for wastewater treatment of only 0.84%, corresponding to 0.19 Euro/pe/a. Specific costs for nitrogen removal (0.32 Euro/kgN) are comparable with other biological processes for N-removal in reject water.

The influence of temperature and SRT on high-solid digestion of municipal sewage sludge

The influence of temperature and solids retention time (SRT) on high-solid digestion of municipal sewage sludge was investigated in laboratory-scale reactors. Digestion with high-solid concentration reduces the required digestion volume and is advantageous for urban areas. The experimental conditions comprised total suspended solids (TSS) in digested sludge between 4.0 and 4.6%, temperatures in a range of 33 to 41 °C and the SRT between 10 and 25 d. High-solid digestion operates with increased NH4-N concentrations released from organic compounds. The anaerobic process can be limited by high NH4-N concentration and toxic NH3. In this study a stable digestion was observed up to 2,000 mg L(-1) NH4-N and 75 mg L(-1) NH3. Volatile suspended solids (VSS) and chemical oxygen demand removal was 53% and 57% respectively. However, digestion with 10 d SRT led to a declined VSS removal of 49%. The removal at 41 and 37 °C showed minor differences, while reduced NH4-N release and reduced methane production were observed at 33 °C. For economic reasons, high-solid digestion at 41 °C is not recommended, but will not impair VSS removal. The outcomes of this study confirm that digestion with up to 7.8% TSS in the feed is feasible for the tested temperatures and SRT down to 15 d.

Anaerobe biologische Sulfatentfernung aus Industrieabwässern am Beispiel einer Viskosefabrik

KurzfassungIm Rahmen eines vierjährigen Forschungsprojektes wurde am Institut für Wassergüte, Ressourcenmanagement und Abfallwirtschaft, Fakultät für Bauingenieurwesen, Technische Universität Wien die grundsätzliche Brauchbarkeit des anaeroben Reinigungskonzeptes zur Behandlung ausgewählter Abwasserströme aus einer Zellstoff-Viskosefaserfabrik in Labor-bzw. Pilotversuchen überprüft. Ziel der Untersuchungen war die Ermittlung der möglichen Belastungskennwerte und der tolerierbaren H2S-Konzentrationen, um einen stabilen Prozess und eine optimierte Sulfatentfernung zu gewährleisten. Da im zu behandelnden Abwasser die CSB-Fracht für eine vollständige Sulfatentfernung limitiert war, wurde angestrebt, die sulfatreduzierenden Bakterien bei der Substratkonkurrenz mit den Methanbakterien durch die Steuerung der Prozessbedingungen zu unterstützen. Die Untersuchungsergebnisse lieferten die Auslegungsdaten für die großtechnische Anlage zur Sulfatreduktion.SummaryLong term lab-scale experiments were performed at the Institute for Water Quality, Resources and Waste Management, at the Faculty of Civil Engineering, Vienna University of Technology to investigate the feasibility of the anaerobic process to treat wastewater from a pulp and viscose fibre industry. In particular, the aim of the investigations was to evaluate the influence of the free sulphide concentration on COD and sulphate removal efficiency and on the substrate competition between sulphate reducing and methanogenic bacteria. Since the wastewater did not contain enough COD for complete sulphate removal it was of main interest to determine favourable process conditions to steer the substrate competition in favour of sulphate reduction. Further experiments in bench-scale permitted to evaluate applicable COD-loading rates and gain fundamental information about process stability and optimization for large scale implementation.

Energy consumption of agitators in activated sludge tanks – actual state and optimization potential

Depending on design capacity, agitators consume about 5 to 20% of the total energy consumption of a wastewater treatment plant. Based on inhabitant-specific energy consumption (kWh PE120-1 a-1; PE120 is population equivalent, assuming 120 g chemical oxygen demand per PE per day), power density (W m-3) and volume-specific energy consumption (Wh m-3 d-1) as evaluation indicators, this paper provides a sound contribution to understanding energy consumption and energy optimization potentials of agitators. Basically, there are two ways to optimize agitator operation: the reduction of the power density and the reduction of the daily operating time. Energy saving options range from continuous mixing with low power densities of 1 W m-3 to mixing by means of short, intense energy pulses (impulse aeration, impulse stirring). However, the following correlation applies: the shorter the duration of energy input, the higher the power density on the respective volume-specific energy consumption isoline. Under favourable conditions with respect to tank volume, tank geometry, aeration and agitator position, mixing energy can be reduced to 24 Wh m-3 d-1 and below. Additionally, it could be verified that power density of agitators stands in inverse relation to tank volume.

Grundlagen anaerober Prozesse

Im Kapitel 2 werden zunachst die mikrobiellen Grundlagen der Methangarung und anschliesend die chemischen Grundlagen anaerober Prozesse vorgestellt. Insbesondere werden im mikrobiellen Teil die thermodynamischen Grundlagen, die anaerobe Nahrungskette und auch die Einflussfaktoren auf die Methangarung beschrieben. Die gesamte Biomasse auf der Erde entsteht durch Kohlendioxid-Fixierung bei der Photosynthese von Pflanzen und aquatischen Mikroorganismen. Sie dient Tieren oder dem Menschen als Nahrungsquelle. Die Re-Mineralisation pflanzlicher und tierischer Reststoffe zu CO2 erfolgt durch Mikroorganismen und kann unter aeroben und anaeroben Bedingungen stattfinden. Unter aeroben Bedingungen werden abgestorbene pflanzliche und tierische Reststoffe mit Sauerstoff zu Kohlendioxid und Wasser mineralisiert, wobei auch Ammonium und Sulfid hauptsachlich aus dem Eiweisabbau und Phosphat aus dem Abbau phosphathaltiger Zellkomponenten freigesetzt werden. Unter anoxischen bzw. anaeroben Bedingungen, wenn kein Sauerstoff als terminaler Elektronenakzeptor fur Reduktionsaquivalente zur Verfugung steht, werden die Elektronen auf andere Elektronenakzeptoren wie z. B. Nitrat, Eisen, Mangan, Sulfat oder Carbonat ubertragen. Sind unter anaeroben Bedingungen auch diese anorganischen Elektronenakzeptoren nicht verfugbar, werden die Elektronen auf organische Zwischenprodukte des Stoffwechsels ubertragen und es entstehen Garprodukte wie z. B. Milchsaure aus Pyruvat oder fluchtige Fettsauren und Alkohole aus Acetat. Diese Garprodukte sind „reduzierter“ als ihre aus dem Stoffwechsel stammenden organischen Elektronenakzeptoren. Garungen werden nach den dominierenden Abbauprodukten benannt, so z. B. Milchsauregarung bei der Yoghurt-, Buttermilch- oder Silageherstellung, alkoholische Garung bei der Bier- oder Weinbereitung bzw. der technischen Alkoholherstellung und Propionsauregarung im Pansen von Rindern, in Anaerobreaktoren (Faulbehaltern) von Klaranlagen oder in industriellen und landwirtschaftlichen Biogasanlagen. Garungen sind wichtig fur die Haltbarmachung von Lebens- und Futtermitteln, sind aber auch die entscheidenden Prozesse fur den anaeroben Abbau von Biomasse zu Sumpf-, Faul- oder Biogas im Anschluss an die Hydrolyse von pflanzlichen oder tierischen Biopolymeren. Bei einigen Garungen, z. B. der Buttersauregarung wird neben Butyrat und CO2 bereits molekularer Wasserstoff gebildet und ausgeschieden. Weiterer Wasserstoff entsteht bei der Acetogenese von Propionat, Butyrat, und hoheren Fettsauren, die aber nur bei sehr niedrigem Wasserstoffpartialdruck zu Acetat und CO2 umgesetzt werden konnen. Fur einen vollstandigen Abbau von z. B. Butyrat mussen Methanbakterien den Wasserstoff mit CO2 und auch das Acetat zu Methan umsetzen. Wasserstoff hat uber den Partialdruck eine zentrale Regelfunktion fur den anaeroben Abbau organischer Substanzen zu Biogas.

The first two years of full scale operation of the two-stage Main Wastewater Treatment Plant of Vienna.

The Main Wastewater Treatment Plant of Vienna had to be extended to guarantee sufficient nitrogen removal. After intensive evaluation studies a two-stage activated sludge system was chosen for the plant-extension. Due to the very small specific reactor tank volume of two-stage treatment plants in comparison with low loaded single-stage plants internal cycles had to be applied to ensure sufficient nitrogen removal. Starting in the year 2000 the plant extension was finished by the beginning of 2005 and the extended plant went into operation in spring. The Austrian effluent standards had to be fulfilled by December 2005. This paper presents a description of the plant layout and gives an overview of the operating results of the first two years of full scale operation. Mass balances were used to evaluate the pathways of nitrogen removal. The results confirmed the expected flexibility of the chosen concept; nitrogen elimination did not decrease although the effluent recirculation that has been implemented to ensure sufficient nitrogen removal efficiency especially during winter had been reduced markedly due to energy reasons. The treatment efficiency that was observed at pilot plant investigations was clearly exceeded.

Modelling of enhanced CSO treatment in secondary clarifiers with a modified Activated Sludge Model no. 3.

An alternative approach for combined water treatment as opposed to its CSO discharge into receiving water is its bypass to the inlet of secondary clarifiers (SC). To analyse the processes and to evaluate the performance of this approach, experiments and numerical modelling were carried out. In batch and pilot scale experiments major effects were identified and quantified. The Activated Sludge Model No. 3 (ASM3) was modified to simulate the batch and pilot scale experiments for implementation of the bypass-specific processes and thus to set up an overall balance of the relevant compounds. With some modifications of ASM3, good agreement of the modelling results with measurements of COD, nitrogen and phosphorus were achieved.

Rheology and friction loss of raw and digested sewage sludge with high TSS concentrations: a case study

High total suspended solids (TSS) digestion of municipal sewage sludge reduces the required space and volume for digestion plants. However, an important consequence of high TSS is the major influence on sludge rheology. The present case study investigates the rheology of sludge from a 130 m3 high solids digestion pilot plant at Viennas main wastewater treatment plant (4 M PE120). Raw sludge ranged from 6 to 8% TSS and digested sludge from 3.2 to 4.6%. TSS show an exponential impact on rheological parameters. Increasing raw sludge TSS from 6 to 8% at least doubles the shear stress and increases friction loss by a factor of three. However, under real operating conditions simulated at the pilot plant, there are additional impact factors. The mixing ratio between waste activated and primary sludge influences raw sludge rheology, while solids retention time and loss on ignition affects digested sludge rheology. Nevertheless, friction loss calculations based on a simple power law relationship between shear rate and shear stress proved to be applicable and sufficiently accurate for both raw and digested sludge with high TSS. Altogether, this case study underlines the relevance of comprehensive rheological considerations, measurements and calculations when designing high TSS sludge digestion.