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Large wastewater treatment plants (>50,000 population equivalents) treat more than 80% of the wastewater treated on a global scale, today it might be even >90%. They therefore provide the most relevant contribution to water protection from urban and industrial wastewater. This was already the case in 1971 when academics realised that progress in the scientific community alone will not succeed in a rapid transfer of research results to practitioners in design and operation of these plants. At the same time, it was recognised that urgent problems in practice are not recognised early enough by the globally networking scientific community. The most effective means of solving these problems was the creation of a new forum where experts from both sides meet. Scientists normally create their special conferences and workshops to enhance global co-operation in their specific field of research and development. This is reflected in the existence of many IWA Specialist Groups (SG) with global representation. The IWA Large Wastewater Treatment Plants (LWWTP) events and the formation of the LWWTP Specialist Group have been the first to take care of a complex technology for water protection, where specialists from most other SGs can meet and discuss together with the practitioners designing and operating treatment plants. In fact, many new Specialist Groups had their origin in the workshop series starting in 1971 in Vienna, taking care of many specific problems reported from practice. The managers and chief operators of large plants, usually only served by meetings of national professional associations, got a new opportunity for global exchange of knowledge and experience together with the global network of scientists and researchers.

The actual mathematical models describing global climate closely link the detected increase in global temperature to anthropogenic activity. The only energy source we can rely on in a long perspective is solar irradiation which is in the order of 10,000 kW/inhabitant. The actual primary power consumption (mainly based on fossil resources) in the developed countries is in the range of 5 to 10 kW/inhabitant. The total power contained in our nutrition is in the range of 0.11 kW/inhabitant. The organic pollution of domestic waste water corresponds to ∼0.018 kW/inhabitant. The nutrients contained in the waste water can also be converted into energy equivalents replacing market fertiliser production. This energy equivalent is in the range of 0.009 kW/inhabitant. Hence waste water will never be a relevant source of energy as long as our primary energy consumption is in the range of several kW/inhabitant. The annual mean primary power demand of conventional municipal waste water treatment with nutrient removal is in the range of 0.003–0.015 kW/inhabitant. In principle it is already possible to reduce this value for external energy supply to zero. Such plants should be connected to an electrical grid in order to keep investment costs low. Peak energy demand will be supported from the grid and surplus electric energy from the plant can be is fed to the grid. Zero ‘carbon footprint’ will not be affected by this solution. Energy minimisation must never negatively affect treatment efficiency because water quality conservation is more important for sustainable development than the possible reduction in energy demand. This argument is strongly supported by economical considerations as the fixed costs for waste water infrastructure are dominant.

China has achieved significant progress on wastewater treatment and aquatic environmental protection. However, leakage (in- and exfiltration) of sewer systems is still an issue. By using the statistical data of water and wastewater in 2016 in China, and the person loads (PLs) of water and wastewater in Singapore, the leakage fractions of hydraulic flow, organic carbon (COD), nitrogen (N) and phosphorus (P) mass loading, and in-sewer COD biological removal in the sewer systems of China (except Hong Kong, Macau and Taiwan), Shanghai, Guangzhou and Beijing were reported for the first time. The fractions of hydraulic flow infiltration (13%, Shanghai and Guangzhou) and exfiltration (39%, China) were calculated. Except Beijing, whose sewer networks are under appropriate management with small leakage fractions, the exfiltration fractions of COD (including in-sewer biological COD removal) ranged from 41% (Shanghai) to 66% (China) and averaged 55%; N ranged from 18% (Shanghai) to 48% (China) and averaged 33%; and P ranged from 23% (Shanghai and Guangzhou) to 44% (China) and averaged 30%. The exfiltrated sewage, COD, N and P not only wastes resources, but also contaminates the aquatic environment (especially groundwater) and contributes to ‘black and odorous water bodies’. In- and exfiltration in the sewer network leads to low influent COD concentration, C/N ratio and high inorganic solids and inert particulate COD concentrations of many municipal wastewater treatment plants (WWTPs) causing high cost for nutrient removal, poor resource recovery, additional reactor/settler volume requirement and other operational problems. Therefore, tackling sewer leakage is of primary importance to today's environment in China. Recommendations for the inspection of sewer systems and the rehabilitation of damaged sewers as well as the development of design and operation guidelines of municipal WWTPs tailored to the specific local sewage characteristics and other conditions are proposed.

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 Vienna's 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.

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.

The successful strategy for water protection by biological wastewater treatment results in a sludge production of about 20 to 40 kg dry matter per population equivalent and year. In the context of regional material fluxes, sewage sludge has a low resource potential and a low pollution potential despite the fact that it can be characterised as a sink, concentrating wastewater compounds. The mass flow of nutrients (N, P) in the sewage sludge is comparatively small as compared to the losses of nutrients in agriculture. The most valuable element in the sludge is phosphorus, as the availability of phosphorus for the production of low-cost mineral fertiliser is limited. The most economical means of P-recycling is agricultural sludge application, which can also be seen as the option with the lowest loss of all valuable compounds of the sludge, and the lowest increase of entropy. The reliability of this disposal route for the treatment plant operator depends on several major pre-requisites:reliability in regard to hygienereliability in regard to long-term soil protectionpublic acceptance (politicians, media)acceptance by all parties involved in sludge application and its consequences (farmers, farmer unions, land owners, food industry, food trade, retailers, consumers, consumer associations, NGOs, etc.) Only the first two prerequisites can be based on scientific research and risk assessment. As a consequence, only for these two aspects can quality criteria and adequate procedures be developed and introduced into a legal framework and quality assurance procedures. The latter two pre-requisites must be addressed in terms of sociology, psychology and political science and have a strong educational aspect.

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.

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 ∼ 6d; 36 °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 ∼ 45% of total nitrogen in digested sludge can be removed. This significantly improves nitrogen removal efficiency at the wastewater treatment plant. NH4-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.

This paper investigates the costs of wastewater treatment (including sludge management) within the Danube catchment countries A, CZ, SK, HU, SL, RO, BG and UA. TK is considered as well. Additionally, the paper compares the total costs of wastewater management (including sewerage) with the incomes in the different countries. The annual costs of wastewater treatment in Austria are about 30 €/p.e. y for large plants with nitrogen and phosphorus removal. In low income countries of the Danube and Black Sea catchment areas they are at a maximum 30% lower than in Austria. However, the incomes in countries like Bulgaria, Romania or Ukraine are 85% to 90% lower. The total annual costs for wastewater management (sewer development plus treatment) amount at least to 90€/p.e. y. Considering the level of income in those countries, financing of wastewater management completely by charges of the population equivalents connected is not feasible. Therefore other approaches for financing wastewater treatment are required.

During a six-year period the Austrian Benchmarking System was developed. The main objectives of this benchmarking system are the development of process indicators, identification of best performance and determination of cost reduction potentials. Since 2004 this system is operated via an internet platform and automated to a large extent. Every year twenty to thirty treatment plants use the web-based access to this benchmarking platform. The benchmarking procedure comprises data acquisition, data evaluation including reporting and organised exchange of experience for the treatment plant managers. The process benchmarking method links the real costs with four defined main processes and two support processes. For wastewater treatment plants with a design capacity >100,000 PE these processes are further split up into sub-processes. For each (sub-) process the operating costs are attributed to six cost elements. The specific total yearly costs and the yearly operating costs of all (sub-)processes are related to the measured mean yearly pollution load of the plant expressed in population equivalents (PE110: 110 gCOD/d corresponding to 60 g BOD5/d)). The specific capital costs are related to the design capacity (PE). The paper shows the benchmarking results of 6 Austrian plants with a design capacity >100,000 PE representing approximately 30% of the Austrian municipal wastewater treatment plant capacity.