Treatment Processes

Fermentation wastes may be treated on-site or at an STW by any or all of the three following methods:

1. Physical treatment.

2. Chemical treatment.

3. Biological treatment.

The final choice of treatment and disposal processes used in each individual factory will depend on local circumstances.

Treatment processes may also be described in the following manner:

L. Primary treatment; physical and chemical methods, e.g. sedimentation, coagulation etc.

2. Secondary treatment; biological methods (e.g. activated sludge) conducted after primary treatment.

3. Tertiary treatment; physical, chemical or biological methods (e.g. microstrainers, sand filters and grass plot irrigation) used to improve the quality of liquor from previous stages (Forster, 1985).

4. Sludge conditioning and disposal; physical, chemical and biological methods. Anaerobic digestion is often used to condition (make it more amenable to dewatering) the sludge produced in previous stages. Following dewatering (e.g. by centrifugation using a decanter centrifuge) the sludge can then be disposed of by incineration, landfilling, etc.

Physical treatment

The removal of suspended solids by physical methods before subsequent biological treatment will considerably reduce the BOD of the resulting effluent. In nearly all fermentation processes the cells are separated from the liquid fraction in recovery processes (Chapter 10). Obviously, biomass processes need not be considered. Yeast cells from other processes may be a marketable product, but microbial cells may not always be marketable, particularly when contaminated with filter aid. In these instances, when the cells and filter aid are a waste, the recovered material may be dealt with in two basic ways:

1. The waste is disposed of without any further treatment.

2. The waste bulk is reduced by mechanical dewatering with a filter press, centrifuge, rotary vacuum filter or belt press. The compressed waste is then incinerated (Grieve, 1978) or disposed of in a landfill site.

Solid wastes are produced in some processes before inoculation. In breweries, where malted grain is still used, coarse screens or 'whirlpool' centrifuges may be used to remove spent grain from the wort after it is mashed. About 5 kg (wet weight) of grain are produced per barrel (180 dm3) of beer. If hops are used, rather than hop extracts, they will also be recovered on screens in a 'hop back'. This residue may amount to 250 g per barrel. Both the spent grain and hop waste may then be mechanically dewatered before being sold or dumped.

The stillage (after distillation) in whisky distilleries may be passed through screens (1 mm openings). These screenings are then removed, mechanically dewatered, and dried in rotary driers to yield a potentially marketable residue known as Distillers' grains. According to a survey in Scotland, about half the whisky distilleries were evaporating the spent waste to a syrup containing 45% solids, mixing with spent grain, drying and selling the final product, 'Distillers' Dark Grains', as a low-grade cattle food (Mackel, 1976).

Physical processes installed for primary effluent treatment may include the following stages:

1. Screens, to remove larger suspended and floating matter.

2. Comminutors, to reduce particle size.

3. Constant velocity channels (~ 0.3 m s"') for grit removal to prevent damage to plant in later processes.

4. Sedimentation tanks for the removal of finer suspended matter. These are generally circular or rectangular continuous flow tanks operating at retention times of 6-15 hours (and designed to have a minimum retention time of 2 hours), with facility for the continuous removal of settled sludge. Sedimentation tanks can remove 70% of the incoming suspended solids and, depending on the nature of the waste, up to 40% of its BOD load (Forster, 1985). They can be operated with or without prior chemical coagul-ation/flocculation. Similar settlement processes are also conducted after secondary (biological) treatment.

Physical processes used in tertiary treatment to produce an effluent of better quality than the 30:20 standard include microstrainers, slow sand filters, upflow sand filters and rapid gravity sand filters. Throughputs vary between around 3 m3 m"2 day"1 for slow sand filters and 700 m3 m"2 day"1 for microstrainers. Suspended solids removal is generally 50-70% and BOD removal around 30-50%, depending on the technique used. A detailed description of tertiary treatment is given by Truesdale (1979) and Viessman and Hammer (1993).

Chemical treatment

Fine suspended particles in an effluent may be removed by coagulation and/or flocculation (Cooper, 1975; see also Chapter 10). Coagulation is essentially instantaneous whereas flocculation requires some more time and gentle agitation to achieve 'aggregation' of the particles. Ferrous or ferric sulphate, aluminium sulphate (alum), calcium hydroxide (lime) and polyelec-trolytes are often used as chemical coagulants. A solution of coagulant of the appropriate strength for effective treatment is added to the effluent in a vigorously mixed tank, a precipitate or floe forms almost immediately and carries down the suspended solids to form a sludge. This sludge may be drawn off, mechanically dewatered and subjected to further treatment. The floes formed on coagulation may be small, and will therefore require an extended period to settle, and as a consequence, for a given throughput of effluent a large sedimentation tank will be needed. Increasing the particle diameter by encouraging small floes to coalesce (flocculation) increases the rate of sedimentation and thus, for a given throughput, a smaller vessel can be operated. Polyelectrolytes are commonly used as floc-culants, and following addition the effluent is gently mixed (turbulent mixing would break up the floes) by passage through sinuous flocculation channels, hydro-dynamic flocculators or mechanically mixed floccula-tors (Smethurst, 1988).

Biological treatment

Most organic-waste materials may be degraded biologically. This process may be achieved aerobically or anaerobically in a number of ways. The most widely used aerobic processes are trickling filters, rotating disc contactors, activated sludge processes and their modifications. The anaerobic processes (digestion, filtration and sludge blankets) are used both in the treatment of specific wastewaters and in sludge conditioning.

Aerobic processes

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