Sludge reduction by predatory activity of aquatic oligochaetes in wastewater treatment plants: science or fiction? A review

Abstract

Biological aerobic wastewater treatment plants (WWTPs) produce a lot of excess sludge. The costs for handling this residual product are increasing, so the search for alternative techniques to reduce the amount of sludge has to be continued. Activated sludge consists of inorganic and organic substances, bacteria, protozoa and metazoa. Due to incomplete biomass conversion, sludge consumption yields less oligochaete biomass. From a technological point of view, the application of aquatic oligochaetes to reduce the sludge production offers interesting perspectives. This paper aims to review the feasibility for the reduction of activated sludge in WWTPs by means of aquatic oligochaetes. Also the current techniques concerning sludge reduction are taken into account. Several of the WWTPs relevant parameters, which may influence predatory activity of aquatic oligochaetes, are discussed: particle size, organic content of substrate, bacteria preference, life cycle and population dynamics of aquatic oligochaetes, temperature, pH, oxygen and process conditions. From the literature it appeared that most research has been performed on laboratory scale. Only a few authors mention a significant reduction of the sludge production by 'sessile' species such as Lumbriculus. Vermicultures for the reduction of activated sludge are rather common in developing countries. Incidentally large annelid blooms have been noticed in WWTPs. It remains obscure which factors trigger the initiation of annelid blooms in WWTPs and which are of importance to maintain a stable annelid population in WWTPs. The influence of a considerable worm bloom on the waste sludge production is still under investigation.

A Novel Biological Method for Sludge Volume Reduction by Aquatic Worms

“…In this research, the environmental factors affecting the performance of worms were surveyed. This research confirmed the use of worms as a protein-supply source in the food for fish and domestic animals [12]. The result of a study about mass balances and processing of worm faces showed that 39% of nitrogen and 12% of phosphorus in the sludge which were digested by the worm were used in the formation of new worm biomass, which had potential for reuse [13].…”

“…The result of this research (30%) was almost equal to that (33%) of the present research in which the worm species and the treatment method were applied in the process (activated sludge) [12].…”

Physical and biochemical changes in sludge uponTubifex tubifexpredation

“…A special case of excess sludge reduction is by means of aquatic worms that naturally inhabit aerobic zones in WWTPs [11]. Several different sessile and free swimming oligochaete worm species have been investigated with a variety of reactor designs for their ability to degrade sludge (see Table 1).…”

Elucidating the microbial community associated with the protein preference of sludge-degrading worms

“…In this perspective, the removal of proteins from the extracellular polymeric substances (EPS) matrix, during sludge predation, could be due to the consumption of bacteria residing in the EPS. The removal of bacteria from ingested particles, which can be referred to as 'microbial stripping' [18], results in changes in microbial community of the natural sediments the worms inhabit [19]; or in case of sludge reduction, changes in the microbial community of the sludge reduction system the worms inhabit [20].…”