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Different treatment routes have been studied for a mixed sludge: the conventional agricultural use is compared with the thermal oxidation processes, including incineration (in gaseous phase) and wet air oxidation (in liquid phase). The interest of a sludge digestion prior to the final treatment has been also considered according to the two major criteria, which are the fossil energy utilisation and the greenhouse effect gases (CO2, CH4, N2O) release. Thermal energy has to be recovered on thermal processes to make these processes environmentally friendly, otherwise their main interest is to extract or destroy micropollutants and pathogens from the carbon cycle. In case of continuous energy recovery, incineration can produce more energy than it consumes. Digestion is especially interesting for agriculture: according to these two schemes, the energy final balance can also be in excess. As to wet air oxidation, it is probably one of the best ways to minimize greenhouse effect gases emission.

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Prevention of pollution due to stormwater flow is the new challenge in water pollution control. With a view to avoid heavy investments in buffering capacity, a new process called Actiflo® has been developed. This process allows an upflow velocity of 130 m/h over the lamella, which means a total area of only 0.03 m2 per m3/h of influent capacity. Under these conditions, a reduction of 80% of the TSS is achieved using about 60g/m3 of ferric chloride and 0.8 g/m3 of polyelectrolyte. High compactness and quick start up of the process allows one to set up the Actiflo® in any place where prevention of stormwater flow is suitable, with low operating and investment costs compared to conventional solutions.

Incineration of sludge is occasionally accused of pollution. This paper shows that if it is correctly designed and implemented, it can be environmentally friendly. For this purpose, sludge incineration is compared to agricultural spreading of limed sludge with respect to toxicity criteria, greenhouse effect gases (GEG) release, energy wasting and other environmental parameters. Landfilling is also considered but as a standby route. Since present regulations on agricultural use and gas emission release from incinerators are stringent, incineration cannot be suspected to release more noxious substances in the environment than agriculture. A distinction is made between biogenic CO2 and fossil CO2. Nevertheless case studies show that incineration produces more GEG and wastes more energy than agricultural spreading if no energy is recovered from hot flue gas. In the case of thermal power or electrical power generation, the environmental balance becomes dramatically more favorable for incineration.