In the reorganization all the wastewater functions in the Helsinki Metropolitan Area were put together and Wastewater Division inside HSY was formed. Sludge composting is done in different ways in Espoo and Helsinki due to existing contracts. From 2013 on, all sludge will be composted in HSY composting area at Sipoo and delivered to market in soil and compost products.

Soil product markets are developing well. Currently the market demand is higher than we actually can produce. General economic situation can naturally affect the soil product demand. The product we are currently producing is of very high quality. It is an excellent product for all kind of green building applications like gardens and parks. Customer feedback is very positive.

Part of our composted biosolids are used in agricultural production. By that that we recycle valuable nitrogen and phosphorus back to food production. Farmland is often poor in humus, which affects farming properties of the land. Compost products contain high amount of organic material. Usage of compost products in farmlands is improving the physical properties of the land, which has positive effect on the growth of the crops. Compost is low in heavy metals but the latest concern are the organic micropollutants in sludge products. Nutrient recycling is necessary in the long term since there are limited resources of phosphorus in the world. The clear positive effects sludge has on the farmland have to be evaluated against possible negative effects of the micropollutants while making decisions on farmland use of sludge products.

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Major part of our WWTP operations is the treatment and utilization of the generated sludge. We dewater all the sludge and until 1997 it was taken to two landfills, which were fillin up. Without any additional measures, we should have enlarged the landfills – especially with the new South-Western (SW) treatment plant to be constructed. Vodokanal surveyed the market and world experience of sludge incineration technologies and decided to use French fluidized bed incineration technology with the specific feature of autothermal incineration, thanks to the calorific value of the sludge itself. In 1997 an incineration plant was built to the Central (C) WWTP with four furnaces, each with a capacity of 250 t/day of dry solids (DS). After putting them into operation we had 48% of the whole sludge amount incinerated, and the volume of sludge to be treated was decreased tenfold. With the ash there is no odor or nutrient leaching problems and it is transported to landfills. We could furthermore produce heat power needed for the residents of a nearby island.

Thanks to the advantages of sludge incineration we had by 2007 launched two more incineration plants with two furnaces each: at Northern (N) WWTP (186 t DS/d) and at SW WWTP (88 t DS/d). Sludge from the city’s 14 small and mid-sized WWTPs is also delivered to the incineration plants. We achieved 100 % of incineration of our sludge in 2007. The new incineration plants are equipped with steam generators and thus we get electric power for 11 % of our own needs for SW and N WWTPs. Plants fully meet all the requirements of the EC Directive 2000/76 for incineration conditions and emissions of pollutants into the air from waste incinerators. However, the construction of the incineration plants was quite difficult. The C and N incinerators were constructed by a French company and the SW by a German company. There are differences in the flue gas cleaning processes: for C and SW it is wet gas cleaning and for N dry gas cleaning – the latter for the first time for us. They have also different supply systems of the sludge to the incineration.

We are planning to install digesters that will allow us to increase our sludge energy potential – practically that means 40 % more. Then we can cover our energy needs with over 50 % and reduce the dependence of external power resources. This improves the quality of the sludge and reduces its volume.

In our prognosis for 2025 the amount of wastewater to be treated will decrease because of flow meters and recycling. However our “Neva untreated wastewater discharge closure programme” still has 7 % of wastewaters to be collected for treatment. We are also planning to reconstruct and modernize three new WWTPs of 2500 – 120 000 m³/d capacities e.g. in Lomonosov and Metallostroy.

Over the years 4.9 million m³ of sludge was stockpiled in our two landfills, so they are filled up to critical levels. Sludge stored is a heterogeneous mass with c. 23-25% of dry solids and c. 33-35% of minerals, containing inclusions. Obviously, such sludge can’t be used in its initial state; therefore we needed a method for preliminary treatment. After technical and economic comparison the static dewatering of the sludge in geotubes was selected. Geotubes don’t need constructing of facilities or installations of devices and thus spare parts or maintenance breaks are not needed. Dewatering is non-stop until complete removal of the reject water in parallel with biostabilization and geoconsolidation of the solid phase.

Now 10 % of the sludge in our Northern landfill has been storaged in geotubes. There is no rain or winter weather influence, which means no hazards or impacts to the environment and no smell problems. The soil product is dewatered, stabilized, and heavy metals are tightly bound. In 2016, 100% of the old sludge will be treated this way. We have developed ways to use this material for landscaping and to enrich the poor urban soils for green areas. We are planning to release half of Northern landfill territory by 2016 and plant trees there. Rest of the area will be divided in two parts, technical land/soil (for SNiP construction rules) and a reserve area (for sludge storage from WWTPs in special occasions like service works at the sludge incineration plant).

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We started our “Privatinstitut für Klärtechnik” in the early nineties as a spin-off company from the Technical University Hamburg, and in our first two projects we supervised the operation control of two pilot plants, which were owned by the state of Schleswig-Holstein and which were designed to examine new methods of biological nitrogen removal. So from the beginning we had a very close view on practical problems. Many of our WWTP-partners know us for more than 10-15 years, and both of us have learned a lot by such a long and close cooperation on operating WWTPs. We share many different operators’ experiences, and with our scientific background and the assistance of our own laboratory we can provide practical solutions for many difficult operating situations. This is one step beyond classical consulting and designing.

It’s the mixture that matters. Not every question on water treatment needs a scientific answer. But with our scientific background we know the limits of possibility as well as the error possibilities in control systems or human thinking. On the other hand, sometimes we have situations which are very special, i.e. in industrial wastewater treatment, and there might be no solution in our or anybody else’s experience. Then individual research has to be started. Scientific methods are needed in finding the economical and practical applicable solutions.


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