Along with inhabiting organic mud, worms also inhabit biological slimes; they have been found in activated sludge and in trickling filter slimes (wastewater treatment processes). Generally microscopic in size, they can range in length from 0.5 to 3 mm and in diameter from 0.01 to 0.05 mm. Most species have a similar appearance. They have a body that is covered by cuticle, cylindrical, nonsegmented, and tapered at both ends.
These organisms continuously enter wastewater treatment systems, primarily through attachment to soils that reach the plant through inflow and infiltration (I&I). They are present in large, often highly variable numbers, but as strict aerobes they are found only in aerobic treatment processes where they metabolize solid organic matter.
Once nematodes are firmly established in the treatment process, they can promote microfloral activity and decomposition. They feed on bacteria in both the activated sludge and trickling filter systems. Their activities in these systems enhance oxygen penetration due to their tunneling through floc particles and biofilm. In activated sludge processes, they are present in relatively small numbers because the liquefied environment is not a suitable habitat for crawling, which they prefer over the free-swimming mode. In trickling filters where the fine stationary substratum is suitable to permit crawling and mating, nematodes are quite abundant.
Not only do nematodes prefer the trickling filter habitat, but they also play a beneficial role in this habitat. For example, they break loose portions of the biological slime coating the filter bed. This action prevents excessive slime growth and filter clogging. They also aid in keeping slime porous and accessible to oxygen by tunneling through the slime. In the activated sludge process, nematodes play an important role as agents of better oxygen diffusion. They accomplish this by tunneling through floc particles. They also act as indicators of operational conditions in the process, such as low dissolved oxygen levels (anoxic conditions) and the presence of toxic wastes.
Environmental conditions have an impact on the growth of nema-todes; for example, in anoxic conditions their swimming and growth are impaired. The most important condition they indicate is changes in the wastewater strength or composition. Temperature fluctuations directly affect their growth and survival—their population decreases when temperatures increase.
Aquatic flatworms (improperly named because they are not all flat) feed primarily on algae. Because of their aversion to light, they are found in the lower depths of pools. Two varieties of flatworms are seen in wastewater treatment processes. Microtubellarians are more round than flat and average about 0.5 to 5 mm in size, whereas macrotubellar-ians (planarians) are more flat than round and average about 5 to 20 mm in body size. Flatworms are very hardy and can survive wide variations in humidity and temperature. As inhabitants of sewage sludge, they
play an important role in sludge stabilization and serve as bioindicators or parameters of process problems; as noted earlier, their inactivity or sluggishness might indicate a low dissolved oxygen level or the presence of toxic wastes.
Surface waters grossly polluted with organic matter (especially domestic sewage) have fauna capable of thriving in very low concentrations of oxygen. A few species of tubificid worms dominate this environment. Pennak (1989) reported that the bottoms of severely polluted streams can be literally covered with a "writhing" mass of these tubificids.
Tubifex (commonly known as sludge worms) are small, slender, reddish worms that normally range in length from 25 to about 50 mm. They are burrowers; their posterior end protrudes to obtain nutrients (see Figure 8.9). When found in streams, Tubifex are indicators of pollution.
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