Biological sludge liquor treatment at municipal wastewater treatment plants – a review

Separate biological treatment of sludge liquor, produced when dewatering digested sludge at wastewater treatment plants, can be favourable in achieving sufficient nitrogen removal. The treatment has the potential to decrease volume requirements and electrical energy and external carbon consumption. The characteristics of sludge liquor (high temperature, high ammonium concentration, low COD:N ratio) support high autotrophic growth rates. These conditions also favour nitrite accumulation, which makes a short-cut nitrification-denitrification process, i.e. nitritation-denitritation, possible, e.g. in an SBR or in a chemostat. As there is no need for external carbon dosage to achieve around 89 % nitrogen reduction in the nitritation-anammox process, it provides an interesting alternative to the nitritation-denitritation process. However, the very slow-growing anammox bacteria require long start-up periods, sufficient inoculum from other plants and extra knowledge for operators. The nitritation-anammox process can be configured in a one- or two-reactor system in floc-type suspended growth, granular or moving-bed biofilm systems. Today, the floc-based systems in SBRs are the most widely used nitritation-anammox systems in full-scale applications, possibly because most old sludge liquor treatment plants are SBRs. Furthermore, the floc-based system has the lowest electrical energy consumption
and the start-up period can be very short because of the use of inoculation.