Streptomycin spent liquor
Aureomycin spent liquor
up to 2,000,000
done (Fisher, 1977). The range of effluent-disposal methods which can be considered is:
1. The effluent is discharged to land, river or sea in an untreated state.
2. The effluent is removed and disposed of in a landfill site or is incinerated.
3. The effluent is partially treated on site (e.g. by lagooning) prior to further treatment or disposal by one of the other routes indicated.
4. Part of the effluent is untreated and discharged as in 1 or 2, the remainder is treated at a sewage works or at the site before discharge.
5. All of the effluent is sent to the sewage works for treatment, although there might be reluctance by the sewage works to accept it, possibly resulting in some preliminary on-site treatment being required, and discharge rates and effluent composition defined.
6. All the effluent is treated at the factory before discharge.
The simplest way of disposal will be on a sea coast or in a large estuary where the effluent is discharged through a pipeline (installed by the factory or local authorities) extending below the low-water mark. In such a case there may be little preliminary treatment and one relies solely on the degree of dilution in the sea water.
If effluents are to be discharged into a river they must meet the requirements of the local river or drainage authorities. In Britain there is a Royal Commission standard requiring a maximum BOD (5 days) of 20 mg dm"3 and 30 mg dm"3 of suspended solid matter (the 20:30 standard). Stricter standards are often applied, depending on the use of the receiving water, such as a 10:10 standard; in addition, levels of ammoniacal nitrogen may be stipulated. There are, as well, often stringent upper limits for toxic metals and chemicals which might kill the fauna (particularly fish) and flora, e.g. sulphites, cyanides, phenols, copper, zinc, cadmium, arsenic, etc. It is highly unlikely that one would be able to discharge an industrial waste today without some form of pretreatment.
Lagoons (oxidation ponds)
Lagoons, holding ponds, oxidation ponds, etc., may be used by a number of industries if land is available at a reasonable cost. It is a method often used in seasonal industries where capital investment in effluent plant is difficult to justify. The lagoon normally consists of a volume of shallow water enclosed by watertight embankments. Oxidation ponds are typically 1-2 m deep. They can be designed to maintain aerobic conditions throughout, but more commonly decomposition at the surface is aerobic and that nearer the bottom is anaerobic and they are then known as facultative ponds. Oxygen for aerobic degradation is provided both from the surface of the pond, and from algal photosynthesis. Deeper ponds (known as lagoons) are mechanically agitated to provide aeration. Lagoons are simple to build and operate, but are expensive in terms of land requirements. They may be used as the sole method of treatment, incorporating both physical (sedimentation) and biological processes, but the effluent produced may not reach locally acceptable standards. Alternatively they can provide an initial pretreatment or can be used to 'polish' effluent from secondary treatment processes.
Liquid wastes can be applied directly to land as irrigation water and fertilizer when they are claimed to have a number of beneficial effects on the soil and plants. If this method of disposal is to be used, then it is necessary to have a large area of land near to a manufacturing plant in an area of low to medium rainfall. Pipeline costs will often restrict use of this technique. Colovos and Tinklenberg (1962) described the disposal of antibiotic and steroid wastes with BODs of 5000 to 20,000 mg dm"3. These wastes were initially chlorinated to lower the BOD and reduce unpleasant odours and then sprayed on to land until the equivalent of 38 mm of rainfall was reached. This process was repeated at monthly intervals and improved plant growth.
When appropriate, solid wastes may be spread onto land as a fertilizer and soil conditioner. This practice is common with sewage sludges, and Mbagwu and Ek-wealor (1990) report the use of spent brewers grains to improve the productivity of fragile soils. Irrespective of whether the waste is liquid or solid, the concentration of heavy metals and certain organic components will require careful monitoring and control to safeguard the environment and public health.
Disused wells, boreholes or mine shafts may provide an ideal, cheap method for disposal when the volume of waste is limited, the underground strata are suitable and the chances of contamination of water supplies utilized by water authorities are negligible (Zajic, 1971). Melcher (1962) has described the use of wells 500-m deep for the daily disposal of:
Acetic acid 900 kg
Ammonium acetate 900 kg
Sodium acetate 760 kg
Sodium chloride 450 kg
Sodium and ammonium bromide 225 kg
Methanol, xylene, tars and organic compounds
This mixture had a pH of 4 to 5 and a COD of 40,000 to 60,000 mg dm~3 rising to 100,000 mg dm"3 and was pumped into the wells at 50 to 100 dm3 min '.
Careful hydrogeological surveys will be needed to prove that waste disposal in wells will not cause pollution of aquifers and threaten groundwater supplies.
Landfilling is a disposal method for municipal solid waste (MSW) and industrial waste. It utilizes natural or man made voids (e.g. disused clay pits) into which the waste is deposited. Both solid and liquid wastes can be deposited depending on restrictions imposed by the site licence. Strict controls exist on the amount of liquid and toxic materials which can be accepted because of the threat of groundwater pollution if leachate (a liquid having BOD levels up to 30,000 mg dm~3) escapes from the site. Leachate is generated from liquid deposited in the site, water entering the site naturally via precipitation or surface run-off and by anaerobic microbial action as organic matter in the landfill is degraded. Microbial action similar to that in anaerobic digesters leads to the production of landfill gas (LFG) which, being 50-60% methane can, if collected efficiently, provide a useful source of energy (Freestone et al, 1994).
A number of designs exist for the incineration of solid and/or liquid wastes either on site or at a commercial incinerator, including rotary kilns, fluidized beds and multiple hearth furnaces. Combustion temperatures need to be carefully controlled to destroy and prevent the formation of dioxins and furans, formation of which occurs at between 300° and 800°, and total destruction is effected at temperatures above 1000° with a retention time of 1 second. Flue gases from the incinerator require cleaning to remove particulates, acid vapours, etc. using electrostatic precipitators, cyclones and wet scrubbers to comply with local environmental protection standards. Waste disposal by incineration is currently significantly more expensive than landfilling, with costs for the disposal of MSW being $37 tonnefor incineration compared with $10 tonne ~1 for landfilling (Smith, 1993).
Disposal of effluents to sewers
Municipal authorities and water treatment companies which accept trade effluents into their sewage systems will want to be sure that:
1. The sewage works has the capacity to cope with the estimated volume of effluent.
2. The effluent will not interfere with the treatment processes used at the sewage works.
3. There are no compounds present in the effluent which will pass through the sewage works unchanged and then cause problems when discharged into a watercourse.
It is common practice for local authorities to demand preliminary on-site pretreatment before discharge into sewers to minimize the effects of industrial wastes. The actual pretreatment required will depend on the precise nature of the waste and may range from simple sedimentation to complex chemical and biological processes.
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