50 years of design and operation of large wastewater treatment plant conferences. A history of innovation and development.

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.

Upgrading of wastewater treatment plants for nutrient removal under optimal use of existing structures.

Upgrading of wastewater treatment plants under maximum use of existing structures is often an important requirement, but also useful due to a number of aspects. Because of a change in legal effluent requirements, a number of plants in Austria, typically aged 20+ years, were required to be extended. The two stage activated sludge HYBRID-process often provides an interesting design alternative for such plant upgrades, especially in case an anaerobic sludge treatment stage already exists. It provides high nutrient removal capacity at low area demand. The latter is especially important in cases where no or very limited extension area is available making it the key factor to preserve a site for future use. Based on two full stage case studies the adaptation of the plant layout, first operation results and a synthetic cost comparison to a conventional (single stage) plant extension are given.

Sewage-treatment under substantial load variations in winter tourism areas--a full case study.

The sewage-load variations in winter tourism areas are characterized by sudden increases--in the range of a factor two to three--within only a few days at the start and the end of the tourist season, especially at Christmas. The sudden load increases occur during periods of low wastewater temperatures, which is an additional demanding factor with respect to nitrogen removal. A full case study was carried out at WWTP Saalfelden, which is located near one of Austria's largest skiing resorts. The plant is designed for 80,000 PE and built according to the HYBRID-concept, which is a special two stage activated sludge process for extensive nutrient removal.

A new side stream process for easily degradable industrial waste waters to avoid sludge bulking.

A new treatment scheme for the treatment of easily biodegradable industrial waste waters has been developed. The side stream treatment of dairy waste water with the excess sludge from the domestic treatment line of the regional treatment plant Bad Vöslau has been operated successfully for a period of three years during which the industrial load stemming from the dairy increased from 800 kg COD/d to 2,500 kg COD/d with peak loads up to 5,000 kg/d. Despite of the increased load to the treatment plant the total aeration tank volume had not been increased. This treatment is performed in an existing aeration tank of the WWTP (V = 1,800 m3) which is now used as contact tank for the combined aeration of dairy waste water and excess sludge from the domestic treatment line (volume aeration tank = 15,000 m3). In this tank the easily degradable substrate from the industrial waste is mainly adsorbed to the biological sludge and after a mechanical dewatering transferred to the anaerobic digester where it yields in an increased gas production. The filtrate of the dewatering process is completely free from biodegradable material and can without danger of bulking be fed to the aeration tank of the domestic treatment line. The new process has proven to be extremely flexible since already now daily peak loads exceeding the design load by more then 60% could be treated in the plant without any problems. Compared to other alternatives for the dairy waste water treatment that were investigated during this study, the new side stream process is very advantageous. No other pre-treatment process for industrial waste water could have been operated under comparable loading conditions without severe operating problems.

Influence of changes of the wastewater composition on the applicability of UV-absorption measurements at combined sewer overflows.

The applicability of UV-absorption measurements at combined sewer overflows was investigated under different storm water conditions. Three specific rainfall events of different intensity, different duration and different intervals between the events were investigated. An individual evaluation of each single event resulted in an acceptable correlation between the UV absorption and the pollution measured as COD. A combined evaluation of the different events exhibited a very poor correlation which could not be used as pollution control parameter. Since the calibration procedure of the spectrometer used was not flexible enough to distinguish between different wastewater matrices from the different storm events the applicability of this type of spectrometer for the control of storm water overflows is questionable.

Two stage activated sludge plants--influence of different operational modes on sludge bulking and nitrification.

Conventional two stage activated sludge plants often lack sufficient nutrient removal performance due to substrate limitation for denitrification in the second stage. For the extension of the Vienna Main WWTP a two stage concept has been developed and tested by means of a pilot plant (scale 1:10.000). The new concept enables the operation of two different modes: In BYPASS-mode a portion of the primary clarifier effluent is fed directly to the second stage; the HYBRID-mode includes the exchange of mixed liquor between the two stages; over the course of the pilot plant investigations it turned out that nutrient removal is strongly increased in comparison to conventional two stage mode, but the two modes of operation lead to different results with regard to the sludge quality and the nitrification performance. BYPASS mode yields a higher SVI in both stages and a lower nitrification performance in comparison to HYBRID mode. This is caused by the negative influence of the primary effluent on the biocoenosis of the second stage. Additionally, the reduced sludge loading of the first stage in this mode results in a higher sludge age which favours the growth of filaments (Microthrix and Nocardia). In HYBRID-mode the higher load of the first stage results in a lower sludge age, fatty components are metabolized and incorporated in the sludge, thus, the growth of filaments is significantly reduced. Additionally, nitrification inhibiting substances are degraded in the first stage, which results in a higher nitrification performance in the second stage.

Treatment of easily biodegradable wastewater avoiding bulking sludge.

The presence of easily degradable compounds from food industries frequently leads to bulking problems. The paper describes a new process that has been developed for a dairy in Austria. Because of the increase in production the treatment plant receiving the wastewater up to now was not able to handle the increased loads. Therefore detailed studies for treatment alternatives have been undertaken which led to a completely new concept. The excess sludge of the urban treatment plant is contacted with the concentrated dairy waste in a separate contact tank. In this tank the easily degradable substrate from the industrial waste is mainly adsorbed to the biological sludge and after a mechanical dewatering transferred to the anaerobic digester where it yields an increased gas production. The filtrate of the dewatering process is completely free from biodegradable material and can without danger of bulking be fed to the aeration tank. The process has been in operation for more than one year and has fulfilled all expectations.

Comparison of different operational modes of a two-stage activated sludge pilot plant for the extension of the Vienna STP.

A pilot plant has been operated over a period of two years in order to investigate the performance and the operating characteristics of the plant concept developed for the extension of the main Vienna STP and to develop a simulation model which will be applied for operation support of the full stage plant. The pilot plant is a two stage activated sludge plant, each stage comprising of four aeration tanks and a clarifier tank. The pilot plant layout allows three different operational modes, each of which has been operated for several periods. The performance of the pilot plant during these periods is described and the different operational modes are compared to each other.

A new process for integrated treatment of industrial wastewater at municipal wastewater treatment plants.

A new process for integrated treatment of industrial wastewater at a municipal WWTP is presented. The process uses the ability of heterotrophic bacteria of rapid substrate uptake and storage. The excess sludge from the municipal treatment line is fed to a contact tank, which receives the industrial wastewater via a separate pipeline. The contact tank is aerated to such a degree that secures complete substrate uptake but minimises oxygen consumption due to instant substrate degradation. The sludge from the contact tank is mechanically pre-thickened and then fed to an anaerobic digester. Hence, the major portion of the incoming COD load of the industrial wastewater is transferred to the anaerobic digester, where it contributes to an increased biogas production. The back load of the industrial wastewater treatment process to the municipal treatment line is very low.