Alkaline stabilisation is a method to render the sludge unsuitable for the survival of microorganisms by adding alkaline material (usually lime) and raising the pH to 12 or higher. An advantage is that a rich product results with substantially reduced pathogens. This sludge has to be applied in agriculture carefully, for it can disturb the pH in the soil, especially in sandy soils. A disadvantage is that the product mass is increased by the addition of lime and that at falling pH-values the microbial activity can restart and odours can be produced. If the sludge contains high ammonia (NH₄) concentrations, for example after anaerobic processes, the increased pH-value changes the NH₄/NH₃ equilibrium towards NH₃, which has a strong, unpleasant odour.

Among the chemical reactions that take place are the following ones:

Thermophilic anaerobic digestion

Thermophilic anaerobic digestion follows the same biological processes as mesophilic anaerobic digestion. It plays a minor role in Sewage sludge stabilisation because of higher energy cost and bad odor production. However, many activities on thermophilic digestion are currently driven in the US by bans of Class B biosolids land application and requirements of Class A disinfection standards for biosolids (40 CFR Part 503; US EPA, 1993). Thermophilic digestion in full scale is usually run at temperature from 55-60C (Ahring, 1994, van Lier, 1996) where the methanogenic bacteria have a max growing rate.

Thermophilic anaerobic digesters consist (like mesophilic ones) of the following basic components: feedstock storage and handling system, digestion tank, gas and residue recovery systems. The digestion tank requires a mixing system which can either be mechanical or achieved by bubbling the biogas through the organic slurry. The digester can be either above or below ground level and should be insulated. In Northern Europe the digester would be fitted with internal heat exchangers to maintain temperatures close to the thermophilic optimum for the methane bacteria (about 55C). Thermophilic systems offer several advantages, including often higher methane production, faster throughput (smaller hydraulic retention time: 5-10 days), better pathogen removal and the prospect of compost production to a consistent standard. Disadvantages are reported as a more unstable process and more difficulties in operation like the tendency of foaming.

Pasteurization

In order to kill pathogens, pasteurization is a common technologie which can also be applied on Sewage sludge. FAO requires a minimum of 30 minutes at 70°C or minimum of 4 hours at 55° C (or appropriate intermediate conditions), followed in all cases by mesophilic anaerobic digestion (FAO, 1992). But in this case the hygienisation is not guaranteed because pathogens easily regrow in mesophilic conditions. It is more safe to pasteurize at the end of the treatment process.