potential justifies it, mechanical dewatering has been accomplished with vacuum filters, centrifuges, thickeners, vibrating screens, filter presses, or rotary and flash dryers. The use of these methods usually is appropriate when sludge reclamation and utilization as a soil conditioner is an objective. If there is no market for the dewatered sludge, it must be disposed of as landfill, by deep ocean dumping, or by incineration to ash.

Industrial wastes

Cooperation between the community and industry in the solution of waste disposal problems has improved a great deal. Nevertheless, there is a need for continuing research, even though there are many industrial waste problems existing today that are amenable to solution. Where the waste problem is one of toxic chemicals, the solution may be one of disposal elsewhere than the public sewers. Many of the organic wastes are becoming more and more conquerable by biological oxidation methods.

In-plant control. The first consideration in handling any industrial waste problem is to commence the solution at the source of the waste. The industrial processes should be thoroughly studied. If possible, a water balance should be undertaken and efforts made to eliminate needless waste, both in raw materials, quantity of water used and waste products. Often, a valuable byproducts industry can be established through recovery of waste products and their conversion to usable raw materials or marketable products. Large volumes of relatively unpolluted water should be recirculated or used in process phases where water quality is not critical. It may be possible to discharge these unpolluted wastes without treatment to the receiving stream itself, or to storm sewers or ditches. Highly concentrated waste components, if not salvageable, might more profitably be discharged to other disposal outlets, such as incinerators or dumps, rather than the sewers or a stream. Not at all to be overlooked are the substantial reduction in waste production that can sometimes be affected by changes in unit process and substitution of raw materials having a lower pollution potential.

Volume of flow, uniformity of rate of flow, concentration and character of suspended and dissolved solids, temperature, pH, and carbon to nitrogen ratios are factors which determine treatment methods and need to be studied. Whether or not there are any components of the waste likely to be toxic to biological treatment media is likewise important.

Treatment practices. In treating industrial wastes without sewage, it is still possible to utilize mechanical and biological processes familiar in sewage treatment. Screens, particularly of the fine variety, have their application. Sedimentation with or without flocculation, often. aided by chemicals, is used for solids removal. Trickling filters and activated sludge processes are readily adaptable to all forms of organic wastes and many that are toxic. Oxidation ponds and spray irrigation may be applicable to organic wastes, and anaerobic digestion has been used for some concentrated organic wastes. Solids treatment and disposal for organic wastes may utilize the methods commonly used in municipal waste treatment. Some processes which are more commonly used in industrial waste treatment plants than sewage treatment plants include flotation, centrifuging, adsorption, and ion exchange.

Nutrient requirements. Biological treatment of industrial wastes with sewage is usually more readily accomplished than without, because sewage furnishes the necessary nitrogen and phosphorus, which are lacking in many wastes. It is quite important to avoid shock loads of either volume or concentration. These can be overcome by the use of holding tanks to equalize these factors and to permit proportionate mixing of the industrial wastes with the sewage. Pretreatment of the wastes is often desirable, taking the form of screening, sedimentation, or chemical precipitation for solids removal; flotation for oil or grease removal; chemical treatment for pH adjustments; or specific treatment for a waste component which cannot be handled in a biological process.

USE OF NEW MATERIALS, EQUIPMENT, AND DESIGNS AND NEW APPLICATIONS OF OLDER MATERIALS, EQUIPMENT AND DESIGNS

Since disposing of the sludge produced in waste treatment works has been one of the major and most costly problems, considerable effort has been made to improve the processes which have been used, as well as to develop new methods. Much of this work has been concentrated on the digesting and vacuum filtration processes. Sludge digestion

Various devices have been developed to overcome operating problems which have occurred in digesting tanks, particularly scum formation. These devices have been intended basically for agitating the contents of digestion tanks. They have taken the form of both mechanical mixers, with and without draft tubes, and devices for collecting the gas produced and returning it to the tank through perforated pipes or diffusion tubes.

As these mixing devices have come into use and as further research work has been done, it has been found that by (a) mixing heated digesting tanks, (b) feeding the settled raw sludge at a more or less continuous rate, and (c) concentrating the raw sludge before addition to the digester, the size of digesting tanks could be reduced. This is often known as high-rate digestion, since the displacement time is shorter than normal.

Another development for the sludge digestion process which should result in additional economies is the draft tube heat exchanger. (See fig. 7.) An adaptation for the draft tube mixer, either mechanical or gas type, has been developed so that it will function as a heat exchange. A jacket is added surrounding the draft tube through which hot water is circulated. Heat is transferred to the sludge as it moves through the draft tube mixer. This adaptation simplifies the heating operation since it is much easier to circulate hot water rather than sludge. This modification also makes use of hot water boilers and results in lower initial costs as well as reduced operation and maintenance costs.

Vacuum filtration

Advancements have been made in vacuum filtration of sewage sludge, particularly in the development of new filter media. These new media include synthetic materials, such as saran, polyethylene, and dacron; stainless steel coil springs; and woven stainless steel belts. New methods also have been developed for alineing and cleaning the filter media, and discharging the dewatered cake. (See fig. 8.)

FIGURE 7.-Draft tube heat exchanger as manufactured by Walker Process Equipment, Inc.

Instead of applying air pressure to discharge the cake, the media is removed from the drum and taken around a small radius roller before being put back on the drum. This action discharges the cake and allows the media to be cleaned either continuously or at frequent intervals with water sprays before it is returned to the drum.

These developments have increased the efficiency of vacuum filtration to the point where it has been possible to bypass the digestion step and dewater raw sludge. Furthermore, polyelectrolytes have been developed which show promise of making it possible to reduce the quantity of chemicals required to condition the sludge for filtration and to obtain greater production from vacuum filters.

Sludge thickening

Laboon process.-Two new developments in the field of sludge thickening or disposal have been the Laboon process and the RotoPlug sludge concentrator. The Laboon process, which has been installed at Pittsburgh by the Allegheny County Sanitary Authority, consists of preheating fresh sludge, containing about 10-percent solids, to about 95° F. before admission to sludge concentration tanks where the sludge remains for about 5 days. The hot raw sludge solids tend to rise to the surfaces of the tanks, and after withdrawal of the subnatant liquor, the sludge is removed from the bottom of the tanks at a concentration of 15- to 20-percent solids. This sludge can then be incinerated for disposal.

Roto-Plug sludge concentrator.-The Roto-Plug sludge concentrator is a device which will thicken or dewater raw primary sludge without chemical conditioning. Digested and perhaps secondary sludges_will require the addition of fibrous material such as paper pulp. It is composed of a thickener unit and a compression unit. The thickener is a hollow cylindrical unit having a peripheral nylon filter media and flanges at the ends. This drum is rotated as the sludge is fed into it. A substantial part of the moisture drains through the nylon media. The solids deposited into the media form into a rolling mass, or plug, which further dewaters the sludge as it presses against the media. The media is kept clean as the solids depositing on it are picked off by adhesion to the plug. The plug increases in size as more solids are picked up and distends axially over one end flange to be discharged from the unit. The compression unit then further dewaters the sludge by compressing it between two impervious drums and a drum of slotted stainless steel. The dewatered cake is comparable to that from a vacuum filter, 20- to 25-percent solids, and may be disposed of by sanitary land fill or incineration.

Zimmermann process

Perhaps some of the most significant and revolutionary developments in sludge disposal in the last few years have been the Zimmermann process and a method developed in Canada known as the atomized suspension technique. In the Zimmermann process (see fig. 9), the equipment used includes a reactor, air compressor, heat exchangers, sludge pumps, and an expansion engine.