AerobicStabilisation of Waste Activated Sludges

ReportNo WSAA 127

October 1998




Aresearch program was undertaken to establish the stability of waste activatedsludge generated from ten activated sludge treatment plants, both nutrientremoval and conventional, and determine what further treatment is required toproduce a substantially stabilised (ie. Non-odorous) sludge.


Ithas been previously thought that waste activated sludge from extended aerationplants (sludge age of approximately 25 days) was sufficiently stabilised topermit dewatering and stockpiling without odour generation. However, experienceat a number of treatment plants with large unaerated mass fractions forbiological removal of nitrogen and phosphorus has demonstrated that thesesludges are generally odorous.


Withthe increasing requirement for on-site storage of sludge to remove pathogensprior to sludge re-use, odour generation from secondary sludges has thepotential to pose significant environmental problems for many treatment plants.


Experiencewith processing and stabilisation of biological nutrient removal (BNR) sludgesis limited in Australia. Lime stabilisation appears to be the favoured approachat many BNR plants, but has had variable success. Anaerobic digestion of wasteactivated sludges solely is not viable due to the lower calorific value of thesludge compared with primary sludges, which would require the waste activatedsludge to be digested at ambient temperatures for a longer sludge age, thusincreasing the cost of the system. Anaerobic digestion of combined primary andwaste activated sludges is practised at several plants, but results in nutrientrich sidestreams which may require additional treatment.


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


Aerobicdigestion at higher temperatures such as with autothermal thermophilic aerobicdigestion processes (ATAD) requires that the sludge have a relatively highcalorific value in order to sustain the thermophilic conditions. Hence, thereare very few examples of ATAD operations with solely waste activated sludge.


Theobjective of the research program was to quantify the degree of stabilisationachieved in various activated sludge treatment plants, what additional aerobictreatment is required to achieve a stabilised sludge and what are the readilyidentified characteristics of a stabilised sludge, including volatile solids content,specific oxygen uptake rate and pathogen destruction. The phosphorus leachingcharacteristics were also compared between various sludges and betweencontinuous and intermittent aerobic digestion process.


Itwas concluded that, in general, the initial stability of the biosolids isdetermined by the degree of treatment within the activated sludge process.However, biosolids generated from nutrient removal plants are still typicallyunstable, unless they are operated at sludge ages above 40 days.


BNRsludges are usually less stable than the equivalent sludge age extendedaeration sludge due to a relatively large fraction of the BNR reactor beingunaerated. All the plants operating at sludge ages above 15 days were able tomeet the NSW biosolids SOUR stability criteria of 1.5 mgO2/gTSS.hrafter 7 days aerobic digestion. Generally plants operating at less than 10 dayssludge age produced biosolids that require greater than 7 days aerobicdigestion to meet the same stability criteria.


Itwas observed that intermittent aeration resulted in slightly greater VSSdestruction than continuous aeration.


Ingeneral there was a significant reduction in organic nitrogen (nitrogen presentin cell biomass), particularly with continuous aeration. However, most of theorganic nitrogen, after ammonification, was oxidised to nitrate. Some of thesamples also indicated that there was significant nitrification anddenitrification occurring, even under continuous aeration, supporting thephenomenon of simultaneous nitrification and denitrification.


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


E. coli was used as a measure ofpathogenic 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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