Biological Characteristics Of Aquatic Worms And Influence On Properties Of Sludge In Process Of Sludge Predation

In the recent years, a novel strategy for sludge reduction through aquaticworms’ predation has attracted significant attention. This technology is achieved onthe basis of a food chain from sludge to worms, and has advantages of lower secondpollution and less energy costs. At present, the majority of related researchesfocused on the performance of sludge reduction whereas few studies paid attentionto the worm growth and the influence of worm predation on sludge characteristics.Relatively little is known about the stability of worm population during sludgepredation and the variation in components and properties of sludge such as heavymetal, extracellular polymeric substances and microbial activity. However, all theseinformation are vital to the practical application of the technology.In the present study, aquatic oligochaeta worm Limnodrilus hoffmeisteri wasused as predator for sludge reduction. The worm growth and the influencemechanism of predation on sludge characteristics were further investigated. Thisobjective was accomplished by confirming the optimal habitat for the worms atdifferent development stages, describing the growth trend of worm populationduring sludge predation; analyzing the heavy metal distribution among sludge,supernatant and worms, discussing the variation in heavy metal speciation and thechange in worm activities; and revealing the effect mechanism of worm predationon sludge settling.The results of growth characteristics of worms showed that newborn wormshad relatively more tolerance to the improper process conditions. The suggestedoptimal process conditions for worm growth and sludge predation included a watertemperature of25°C, an initial worm density in the range of11~12g/L, intermittentaeration with intensity of2.8m~3/（m~2·h）, a pH value of6.5, a NaCl concentrationbelow200mg/L, a sludge concentration in the range of2000~6000mg/L, a SRTwithin3days, a carrier thickness of1cm, and the natural light. Under such optimalconditions, the average sludge predation rate was297mg-TSS/L/d, the duration ofworm life cycle was74.6days, and the asymptotic population growth rate of wormpopulation was approximately1during a period of117days, indicating a stablepopulation size in the sludge predation system.The results of sludge components during worms’ predation showed that theworms had no preference to the carbon or nitrogen in sludge and cannot absorb therefractory organics in their bodies, but had a slight accumulation for inorganiccomponent. More than81%of Cu, Zn and Pb remained in the sludge after predationby the worms, while4.1%,9.7%and4.8%of these metals were released into the liquid, and11.9%，8.2%, and10.5%of that accumulated in worms. The predationdid not increase heavy metal bioavailability in the final sludge as reflected by adecrease or unchanged levels in labile metal, while it caused a release of organiccarbon and phosphorus from solid into supernatant.The results of sludge properties during worms’ predation showed that thepredation decreased the SVI value, the zeta potential, the viscosity, and the amountof bound-EPS of sludge, enhancing the cohesion and hydrophobicity of the sludge.A part of protein bound to sludge flocs were stripped out by worms while thesoluble protein were degradation, both of which helped for the sludge conditioningthrough neutralizing the charge and compressing the double electrode layer,resulting in a higher dehydrated efficiency. The predation of worms onmicroorganisms of sludge was random with low impact on the microbialcommunity structure. The final sludge also exhibited remarkable performance ofammonia nitrogen removal and COD removal.The present study gave a systemic analysis for the process of sludge predationby oligochaeta worm L. hoffmeisteri, and demonstrated that this species is an idealpredator for sludge reduction. The growth characteristics of worms can adapt to thelong-term operation of sludge predation, exhibiting stable sludge reductionefficiency and relatively low effects on predation process and environment. Theresults provide a further understanding for this novel technology for sludgereduction and may help for the development of practical application.