Physical and biochemical changes in sludge uponTubifex tubifexpredation

Abstract

Worm predation (WP) on activated sludge leads to increased sludge degradation rates, irrespective of the type of worm used or reactor conditions employed. However, the cause of the increased sludge degradation rates remains unknown. This paper presents a comparative analysis of the physical and biochemical aspects of predated sludge, providing insight into the hydrolytic mechanisms underlying WP. To this end, the sessile worm Tubifex tubifex was used as a model oligochaete and was batch cultivated in an 18-L airlift reactor. Predation on activated sludge showed an average reduction rate of 12 ± 3.8%/d versus 2 ± 1.3%/d for endogenous respirated sludge. Sludge predation resulted in an increased release of inorganic nitrogen, phosphate and soluble chemical oxygen demand (sCOD). The sCOD consisted mainly of polysaccharides; however, fluorescence excitation emission matrix spectroscopy analysis also revealed the presence of Tryptophan-protein-like substances. Results suggest that the released polysaccharides contain a protein-like element. Additionally, soluble iron increased slightly in concentration after WP. The extent of hydrolysis seemed to reach an average plateau of about 40% volatile solids (VS) reduction after 4 days, which is substantially higher than the 29% VS reduction for endogenous decay of activated sludge after 30 days. Furthermore, T. tubifex predominantly consumed the protein fraction of the extracellular polymeric substances. Results suggest that that the worms specifically target a fraction of the sludge that is predominantly biodegradable under aerobic conditions, albeit at significantly higher degradation rates when compared to the endogenous decay of waste activated sludge. ARTICLE HISTORY

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

“…The temperature was 20 ± 1°C and dissolved oxygen was above 5 mg/L. Reactor performance data can be found elsewhere [5].…”

“…tubifex worms were bought from a wholesale supplier (Aquadip b.v. the Netherlands). Details regarding the identification and handling of the worms can be found elsewhere [5]. The general composition of T. tubifex consists as a % of dry matter mostly of protein (60%), lipids (11-33%) and carbohydrate (16%) [30,31].…”

“…The solids removal during worm predation is mainly due to the reduction of the protein fraction in the sludge [5,10] and is accompanied by the release of degradation products, such as phosphate and inorganic nitrogen, but also soluble chemical oxygen demand (COD), which partly consists of polysaccharides and a limited fraction of proteins [5,6,[11][12][13]. The protein reduction can be attributed to the synergistic activity of the oligochaeta and their intestinal bacterial community [14].…”

“…Aquatic worms such as Tubifex tubifex [5], Lumbriculus variegatus [6,7] and Aulophorus furcatus [8] have shown to be highly efficient in reducing sludge solids. Between 20% and 50% of the volatile solids (VS) can be removed through worm predation [5,[7][8][9] in a matter of days.…”

“…Between 20% and 50% of the volatile solids (VS) can be removed through worm predation [5,[7][8][9] in a matter of days. Similar reduction values can be found for aerobic and anaerobic digestion, however in a time frame of 30 days [5].…”