Management of the excess sludge production resulting from biological wastewater treatment is one of the most important economic and environmental issues for the next decade. Due to the intensification of wastewater treatment, a large increase in sludge production from biological processes is inevitable. Moreover, the main solutions for final sludge disposal are facing increasing hurdles – the use of biosolids for agriculture needs strict quality control and landfilling is restricted by the guidelines for high mineral content.

Sludge incineration cannot be a systematic solution because of its cost and
environmental concerns. The stringent regulations regarding sludge treatment and disposal imposed in several countries as well as social and environmental concerns have resulted in an increasing interest in emergent processes aimed at the reduction or minimisation of excess sludge production. In countries with sewage temperatures above 20°C anaerobic pre-treatment allows to reduce the sludge production to 30% of conventional alternatives, cutting down energy use by an equal amount.

The adaptation of anaerobic pretreatment to lower wastewater temperatures is still under way. The other alternative are new integrated strategies for sludge management based on the reduction of the biomass growth rate and on the intensification of sludge mineralisation, both resulting from the synergy between a biological process and the application of a stress, based on a thermal, chemical or mechanical effect. The drive for reduced sludge production was first observed in Japan, where solids disposal cost of close to £109/wet t are common (or with a typical dry solids content of 15%, a disposal cost/dry t of £711/t DS), which allowed even ultimate disposal like vitrification and plasma torches to mineralise solids to be cost-efficient. But another way to reduce sludge disposal cost was to obtain sludge reduction through the association of an ozonation treatment to a conventional activated sludge process.

ozone transfer

Ozone is directly applied on the sludge taken from the biological reactor before returning the ozonated liquor to the aerated tank. High ozone transfer is obtained using a mechanical mixing device, which insures high level of reaction between sludge and ozone. There is a relationship between the ozone dose and the sludge reduction achieved, with each applied g of ozone resulting in up to 6g of solids reduction. But since typical ozone production cost are around 55p/kg O3, the system would be cost-efficient by totally eliminating solids production only if the disposal cost of the solids are above £328/t DS.

On the other hand, the activity of activated sludge also is reduced with higher ozone dosages, with a practical range between 20-40mg O3/g VSS, as the latter dosage leads to a reduction to about 75% of biomass activity. Also, the soluble residual COD in the effluent tends to be higher with the solubilisation of biomass. Nevertheless, there are now a few suppliers in Japan, the leading process called Bioleader having more than 35 references in both industrial and municipal applications, with the oldest installations having more than five years’ experience. In France, a demonstration plant was run for more than one year on an 1,000PE existing activated sludge plant located in Aydoilles. During the experimental period, the global sludge reduction efficiency was 60%, nevertheless higher reduction efficiency (until 80%) can be reached depending on the ozone dosage applied on the system.

Slight decrease in COD removal efficiency (<5%) was observed, although the removal efficiency of suspended solids and ammonia was maintained and even improved (see Table 1). An enhancement in sludge settling and dewatering characteristics were also observed. The ozonation of return sludge can thus be an interesting alternative to avoid any investment and operational cost for solids treatment in small treatment plants. For larger plants, and in order to avoid the negative impact on the main wastewater treatment, the ozonation of anaerobic digestor sludge was tried.

Tests were carried out on two parallel full-scale digestors with a unitary volume of 1,125m3 each between end of 2002 and beginning of 2004. While the HRT in the digestors was 45 days, the digestors achieved historically a reduction of total solids of 54%. With an ozone dosage of up to 50gO3/kg DS, the sludge balance on the treated reactor showed that compared to the untreated digestor, less than 30% dewatered sludge cake was being produced. This was a consequence of the improvement of volatile destruction and a higher solids content in the cake, which reached above 30% DS, compared to the conventional cake of only 20% solids. In parallel to achieving 80% destruction of volatile solids, a proportional increase of biogas production was observed, yielding 30% more biogas with 60% methane content. Whereas the ozonation energy consumption averaged around 0.03kwh/kg of digested solids, the increased biogas energy content is about 0.05kwh/kg. This assumption is based on a biogas yield 0.55m3 methane/kg VS destroyed, a feed volatile content of 85% and a net electricity production/m3 of methane of 0.35kWh/m3 of CH4.

Therefore, unlike in the direct ozonation of activated sludge, which is a net user of energy, the enhancement of anaerobic digestion by ozone would produce net benefits both by releasing more energy and by reducing the amount of solids disposal for final disposal. When analysing the greenhouse gas emissions over the whole solids cycle, the ozone enhanced digestion reduces them by more than 40%. Under Japanese conditions, the operation cost of sludge treatment was also reduced to about half, with an optimal ozone dose at around 26kg O3/t DS, which leads to self sufficiency of the anaerobic digestion system

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