Municipal Waste Water Treatment Plants

Adding a fourth treatment stage to municipal waste water plants

In the next few years, new approaches and expansion options are needed for the cleaning of waste water in municipal waste water treatment plants (WWTP).

The demographic change in cities and municipalities and the related urban expansion will make expansion of many waste water plants necessary. However, this will be problematic for many waste water plants in terms of available space.

Multiple studies of discharge outlets of waste water plants have also shown in the last few years that many municipal waste waters are contaminated with micropollutants and trace elements. These include tiny plastic particles from car tires or packaging and anthropogenic residues such as medication, radiocontrast agents, biocides as well as multiresistant pathogens. These substances are found at the outlet of waste water plants because at many sites the biological cleaning stage is unable to remove all potential contaminants. Therefore, the focus turns to the discussion about the so-called 'fourth cleaning stage' in waste water plants.

A procedure to eliminate trace elements that is well-tried in pilot projects uses an additional filtration stage, for example granulated activated carbon in combination with a sandfilter in the outlet of the plant. This technology can be combined with an ozonation installed upstream of the filter which first breaks down the contaminants into low-molecular compounds. These compounds can later be adsorbed by the activated carbon, which is regenerated after reaching complete load capacity. Elimination rates of up to 90 % for medication can be achieved.

However, ozonation is not suitable for particularly contaminated industrial waste waters since ecotoxic byproducts may be formed during the chemical reaction. If the water contains high bromide levels, for exampe, the risk increases that a large quantity of the potentially carcinogenic bromate forms. Therefore, the particular water must be tested for suitability before ozonation is implemented.

A second approach is the dosing of powdered activated carbon (PAC) upstream of or directly into the aeration tank. Due to the longer retention time of the activated carbon the adsorption time in the circulatory biological system is prolonged and trace elements can be filtered more effectively. This also enables the development of a biofilm on the surface of the activated carbon which outperforms the degradation rate of conventional activated sludge process. It should, however, be taken into consideration that the complete retention of activated carbon powder is not always feasible with sand filters.

To compensate this disadvantage the PAC-technology may be combined with porous membranes, such as microfiltration or ultrafiltration, to so-called membrane bioreactors (PAC-MBR). For this purpose, submersible membrane systems are directly lowered into the aeration tank. Due to the small pore size of the membranes micro plastics and pathogens can be removed.

Only dense membranes, like those used in nanofiltration and reverse osmosis, are able to filter trace elements directly from the waste water without pretreatment. However, the concentrate of the membrane plants must be disposed of separately. Another advantage of this technology is space-saving. The sludge retention enables the operation of the aeriation tanks with higher sludge content and thus saves space within the expansion of the plant technology.