Aerobic Stabilisation of Waste Activated Sludges

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Aerobic Stabilisation of Waste Activated Sludges

Report no. UWRAA 127

October 1998


A research program was undertaken to establish the stability of waste activated sludge generated from ten activated sludge treatment plants, both nutrient removal and conventional, and determine what further treatment is required to produce a substantially stabilised (i.e.. Non-odorous) sludge.

It has been previously thought that waste activated sludge from extended aeration plants (sludge age of approximately 25 days) was sufficiently stabilised to permit dewatering and stockpiling without odour generation. However, experience at a number of treatment plants with large unaerated mass fractions for biological removal of nitrogen and phosphorus has demonstrated that these sludges are generally odorous.

With the increasing requirement for on-site storage of sludge to remove pathogens prior to sludge re-use, odour generation from secondary sludges has the potential to pose significant environmental problems for many treatment plants.

Experience with processing and stabilisation of biological nutrient removal (BNR) sludges is limited in Australia. Lime stabilisation appears to be the favoured approach at many BNR plants, but has had variable success. Anaerobic digestion of waste activated sludges solely is not viable due to the lower calorific value of the sludge compared with primary sludges, which would require the waste activated sludge to be digested at ambient temperatures for a longer sludge age, thus increasing the cost of the system. Anaerobic digestion of combined primary and waste activated sludges is practised at several plants, but results in nutrient rich side streams which may require additional treatment.

Aerobic digestion of waste activated sludges is quite common, but suffers from process failure if the alkalinity is not controlled; the alkalinity being consumed due to nitrification of ammonia released due to ammonification during cell lysis. However, better control of aerobic digestion, particularly with intermittent aeration, offers promising scope for stabilisation of waste activated sludges, particularly from BNR treatment plants.

Aerobic digestion at higher temperatures such as with auto thermal thermophilic aerobic digestion processes (ATAD) requires that the sludge have a relatively high calorific value in order to sustain the thermophilic conditions. Hence, there are very few examples of ATAD operations with solely waste activated sludge.

The objective of the research program was to quantify the degree of stabilisation achieved in various activated sludge treatment plants, what additional aerobic treatment is required to achieve a stabilised sludge and what are the readily identified characteristics of a stabilised sludge, including volatile solids content, specific oxygen uptake rate and pathogen destruction. The phosphorus leaching characteristics were also compared between various sludges and between continuous and intermittent aerobic digestion process.

It was concluded that, in general, the initial stability of the biosolids is determined by the degree of treatment within the activated sludge process. However, biosolids generated from nutrient removal plants are still typically unstable, unless they are operated at sludge ages above 40 days.

BNR sludges are usually less stable than the equivalent sludge age extended aeration sludge due to a relatively large fraction of the BNR reactor being unaerated. All the plants operating at sludge ages above 15 days were able to meet the NSW biosolids SOUR stability criteria of 1.5 mgO2/g TSS. hr after 7 days aerobic digestion. Generally plants operating at less than 10 days sludge age produced biosolids that require greater than 7 days aerobic digestion to meet the same stability criteria.

It was observed that intermittent aeration resulted in slightly greater VSS destruction than continuous aeration.

In general there was a significant reduction in organic nitrogen (nitrogen present in cell biomass), particularly with continuous aeration. However, most of the organic nitrogen, after ammonification, was oxidised to nitrate. Some of the samples also indicated that there was significant nitrification and denitrification occurring, even under continuous aeration, supporting the phenomenon of simultaneous nitrification and denitrification.

Intermittent aeration reduces phosphorus leaching by 50% or more. Redox control of the aeration input may achieve better control in terms of minimising phosphorus leaching and controlling simultaneous nitrification/denitrification (ie. organism respiration).

E. coli was used as a measure of pathogenic content of the biosolids. In general, the E. coli destruction was greater than 95%, unless the biosolids weregenerated from treatment plants operating at very low sludge ages (eg. 4 days).

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