Decentralised solutions sought: Small sewage treatment plants offer many options
Small modular plants for water management have become increasingly popular over the last few years. And there are some very good reasons for this trend. They help provide flexible solutions for future challenges such as demographic changes, climate change and diminishing raw materials. Small sewage treatment plants also offer interesting perspectives for Asia.
(01.06.2010) In principle, the German water industry is in a very good position. 99 per cent of Germany’s population are connected to a drinking water supply and more than 95 per cent are connected to public drainage systems. However, connecting the last remaining farmsteads and very remote groups of houses is becoming increasingly impractical from economic aspects. The situation is compounded by growing uncertainties associated with demographic development – especially in rural regions. The states of former East Germany are particularly affected. More than 1.5 million people have moved to West Germany since reunification. But regions in the old states, such as the Ruhr and Saarland, are also suffering from declining populations.
On the other hand, some economic centres are growing and commuter belts of modern housing estates are being built on their outskirts. And this trend is continuing. In Saxony, for instance, a recent forecast predicts that the population in most administrative districts will decrease by 2020 – quite considerably in some cases. Dresden and Leipzig are likely to be the only areas where the population will increase. Shrinking municipalities, some of which require considerable modernisation, on the one hand, and growing centres that need leading edge infrastructure for new settlements, on the other, require flexible solutions which, however, must not overtax the tight budgets of the municipalities and their citizens. Local water management systems with innovative technologies could make a contribution here.
They could also help overcome future challenges. Climate change for example: among other things, Germany is predicted to experience an increase in heavy rainfall and flooding but also long periods of drought that could lead to water shortages in some regions. Decentralised water management with separate rainwater collection and irrigation can help lessen sudden overloads in the wastewater systems and the sewage treatment plants. In housing estates and residential complexes, using sanitary water several times to flush lavatories, for example, can help save water and close water cycles.
But that's not all. Decentralised water management concepts can also help save dwindling raw materials and generate energy. Biogas can be generated in anaerobic processes and can then be used for cooking, heating, as a fuel, or to generate electricity. In addition, phosphate and nitrogen can also be recovered for use as agricultural fertiliser. This is a forward-looking concept if we consider that worldwide natural deposits of phosphate, used as a base substance to manufacture fertiliser, could dwindle in 50 to 130 years.
If the same money that has been spent on the current water network was to be spent again throughout Germany, the systems would be equipped so that roughly 1.2 billion cubic metres of biogas and 220,000 tons of fertiliser could be recovered from the wastewater, believes Prof. Walter Trösch from the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB). The IGB is one of the 14 Fraunhofer Institutes in the water systems alliance, SysWasser. The aim of this alliance is to develop sustainable water management technologies and to transfer these into commercially feasible applications.
At present, three Fraunhofer Institutes – along with IGB, the Institute for Systems and Innovation Research (ISI) and the Institute for Transportation and Infrastructure Systems (IVI) – are working on two model projects in the German state of Saxony. Here, about 600,000 inhabitants, roughly 14 per cent of the population, dispose of their wastewater via approximately 245,000 private small sewage treatment plants. They have to be brought up to state-of-the-art standard by 2015. To manage this, according to the state's 'Wastewater strategy 2007-2015', small-scale flexible solution will have to be found. They must be inexpensive, insensitive to load fluctuations and also easy to maintain.
What this solution could look like is to be determined in two residential areas with the cooperation of the residents. The municipality of Altensalz (Vogtland), located directly beside the Pöhl dam, has 33 apartment buildings and 55 detached houses. Wastewater is either disposed of through multi-chamber pits where the overflow runs directly into the dam, or via pits with no discharge that are emptied and disposed of along with the faeces disposal system.
The municipality of Streitfeld (Upper Lusatia) has 67 residential buildings, 13 detached houses, two holiday apartments and a hotel. Most of the wastewater is disposed of via multi-chamber pits with grey water seepage. Some of it is also fed to the hotel's sewage treatment system. According to Dr. Harald Hiessl from the Fraunhofer ISI, a survey of the residents showed that in Altensalz, at least, virtually no demographic change has been noticed, as the municipality is located in the middle of a recreational area with a high leisure value and some of the houses are still occupied by several generations. But this is not the case in Streitfeld where most of the population is of retirement age.
Based on the current status of the project, each municipality will require a different solution. In Altensalz, a semi-central sewage treatment plant with anaerobic wastewater treatment, biogas utilisation and the option of recovering nutrients could be the solution. Wastewater from the apartment buildings and, if requested, also from the detached houses would be fed through a sewage system working according to the vacuum principle similar to that used on ships.
In Streitfeld, the residents and agencies responsible for the project are currently more in favour of a decentralised solution. It is possible that there will be a combination of groups of small sewage treatment works for several houses and small sewage treatment plants for separate properties. Decentralised fully biological membrane systems with nutrient removal will be used. It may be worthwhile for the responsible disposal company, Süd-Oberlausitzer Wasserversorgungs- und Abwasserentsorgungsgesellschaft mbH (Zittau), to test contracting as a business model. Fraunhofer ISI has recently proved that this can work with other project partners in the Dahler Feld municipality in Selm (North Rhine-Westphalia). Here, too, in this rural area (29 buildings, about 100 residents) more than 20 multi-chamber pits had to be remediated. The body responsible for wastewater disposal, Lippeverband, has concluded 10-year contracting agreements with the house owners, involving the construction, operation and maintenance of newly installed small membrane bioreactors (MBR). For these services, the property owners pay a fee – on average, 950 euros per annum for an eight-resident system.
Sustainable water management
The Fraunhofer water systems alliance, SysWasser, has been researching alternative water management concepts for several years. The best known project is called DEUS 21 (Decentralised Urban Infrastructure Systems). It includes two model projects in the state of Baden-Württemberg. In the small village of Neurott near Heidelberg a small compact sewage treatment plant with an innovative rotating disk filter is being tested. These dynamic membrane filters consist of a rotating hollow shaft with filter disks attached. The treated water fulfils the EU Bathing Water Directive. In a new development area in Knittlingen (near Karlsruhe), on the other hand, a more comprehensive water management system is in use. Here, rainwater is collected in an underground system of storage channels and, using membrane technology, is treated to produce safe sanitary water that is suitable for personal hygiene (i.e. bathroom use), for use in dishwashers and washing machines, to flush lavatories and to water the garden. It is provided to the boundary of the properties in a sanitary water system that runs parallel to the drinking water system. This gives house owners the option of installing a second piping system when they are designing their house. In addition, wastewater, optionally together with shredded kitchen waste, is fed directly to a decentralized collection point via a vacuum sanitary system. This wastewater, which is highly concentrated because of the vacuum system, is treated anaerobically in a connected bioreactor and the biogas produced here is utilised.
Author: Heinz-Wilhelm Simon, Berlin (Germany)
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