Large floating solids can be removed by vertically stacked bars called "bar screens". They are mounted in channels at the angle of 10-45 degrees with the horizontal, the acute angle on the downstream side. They may be hand cleaned or mechanically cleaned. Opening b/w the bar are ordinarily 1-2 in. the channel should be of a configuration that will allow minimum velocity of 1fps to avoid sedimentation and a maximum velocity of 3 fps to eliminate the possibility of forcing the solids through the bars. For removal of the fine, non-flocculant or non colloidal particles, disk or drum screens may be used. In these screens revolve on horizontal axis and is cleaned by water sprays continuously. The screening medium is stainless steel, manganese bronze, or alloy wire cloth upto 60 mesh in disc screens and higher in some drum screens. Vibrating screens are also available and also advantageous where screenings should be dewatered before disposal.


This step is often combined with sedimentation when the solids are light and tend to coalesce or when chemicals are added to produce flocculent particles. The process takes place in a basin equipped with the means of slow agitation or stirring, which causes the smaller solids to be attracted to the larger particles by mechanical collision or electrochemical phenomenon.

Care must be taken in design not to restrict flow passage from flocculation basin to sedimentation basin, because appreciable increase in velocity will tear the floc. Velocities greater than 1 or 2 fps should be avoided. In chemical mixing high speed turbines or propeller type mixer are used, in circular or square tanks. A detention time of about 10 to 30 seconds with rapid mixing is adequate. Longer detention times may break flocs. The ratio of depth to width is about 2 ratio 1. Chemicals are applied in a predetermined dosage, measured dry, and then dissolved or suspended on the basis of volume or weight. Most flocculating chemicals are available in liquid form, in which case they are fed by pumps.

Removal of fine suspended or flocculating particles is accomplished by means of clarifiers ( also called sedimentation tanks or settling tanks ). In these, the velocity of the waste stream is diminished to 1 fpm or less by dispersal into tanks of large cross-sectional area and held for two or more hours, as a rule. Lower retention times periods are possible with some wastes, and pilot plant or laboratory studies can determine this. The tanks may be circular with the central inlet and peripheral outlet or the reverse. If the tanks are rectangular, the flow may be from the narrow end to the other, though there are some types in which the flow is vertical, with the tanks divided into cells, each with it's own take-off weir. Square clarifiers are also possible with the central inlet and peripheral outlet. Water depths are on the order of 12 to 14 ft, and with the rectangular basins the length-to-width ratio is at least 5:1. Other controlling factors are overflow rate and weir bounding.

Some solids, which are floating because they have low specific gravity or are fibrous in nature, are best to move by floatation. The method is also applied to grease removal. It involves application of air, with the bubbles carrying material to water surface, where it is removed by skimming. Two methods are generally employed pressurized aeration and vacuum. In the former, the waste stream is subjected to aeration from a blower under pressure which may exceed 25 psi. The pressure is then released in a floatation chamber and dissolved air form bubbles which carry suspended particles to the surface. In the vacuum units aerated waste are carried into enclosed chamber, where vacuum is applied to cause air to release to the surface.

Sludge is then thickened by this method, with poly electrolytes used to increase sludge yield.Design criteria are frequently expressed in pounds per square foot of surface per 24 hours range and from 4 to 20 degrees Celsius.

Organic wastes, following solid separation, must be further treated to remove dissolved unstable constituents this may be done by biological oxidation. Three general processes employed are stabilization in large ponds, trickling filters and activated sludge process.

These are large earthen lagoons in which waste is retained for several weeks. The biological system involves algae which by photosynthesis produce oxygen to effect oxidation. The ponds are kept shallow (2.5 to 5 feet) to maintain algae growth through out the depth. The loading is recommended to be around 10 to 50 lb BOD per acre per day for domestic waste. Proper loading for industrial effluents should be determine by experimentation. Raw waste with low suspended solids content may be treated in this manner without initial solid separation. In areas where percolation through soil is a problem, the ponds may be lined by asphalt or by plastic materials.

Multiple cells in series or parallel are frequently used for flexibility of operation, to prevent solid spill and to accommodate varying biota system occurring with the progression of oxidation. Free board should be about three feet allowance for frost heave. A beam of about 10 feet should be provided with an inside slope of 1:4 to 1:7. The dikes should be seeded with short root grass. The shape of ponds should avoid islands peninsulas and coves influent feed is usually less than 1 foot from the bottom.

Employing the aeration units may considerably increase the loading on the ponds and depths, so that one fifth to one tenth land is required. Equipment for the aeration includes both mechanical surface aerators and diffused air systems. Mechanical aerators may be brige type or mounted on floats suspended in place with help of cables. Conventional diffuse air systems may be employed when the configuration of the pond permits baffling to achieve the rolling action required. One system designed specifically for the pond installation involves the use of the perforated plastic tubing snaked transversely across the bottom of the pond, thus forming the series of cells, separated by screen of air bubbles.

Trickling filters In these, bacteria and other microorganisms are cultured for the waste being treated. The devices are beds containing rock or slag in a size range of 2 to 4 in., specifically fabricated plastic media or redwood slats. The waste is distributed evenly over the surface of the beds by a rotary mechanisms. The distributors may be motor driven or hydraulically actuated. The bed is supported by a floor designed to alllw free drainage of the effluent and permit the air passage at all times. The structure is usually concrete, but steel or the tile shells may be used.

The micro organism culture assume the shape of the slime, becomes attached to the media. As the waste percolates through the media in thin films, the culture feeds on the organic matter, reducing it to the stable compounds of nitrogen and carbon. Rock, slag and redwood media are 3-10 ft deep. Plastic media can be much deeper, minimizing the ground area requirement.

In the redwood media, modules of sawed slats, are assembled, with the space b/w slats about 0.7 in. plastic media are made up of corrugated sheets of polystyrene and polyvinyl chloride, which are then assembled into modules for placing.

Recommended loading in terms of pounds of BOD per day per unit volume and ranges from 5 to 300 lb/1,000 cu ft. the effective loading varies to some extent with the waste to be treated. The media must be kept wet all times, and provisions must be made for recirculation of the effluent to compensate for low flows. There is some virtue in recirculation under all conditions, thereby effecting a higher degree removal of organic material. Some humus, which is discharged from the filter from the dead bacterial cells, has to be removed by a final settling tank.

Activated sludge process this method produces the highest degree of the treatment among all treatment facilities. But is sensitive in it's operation and requires strict control. For this reason the trickling filters or stabilization ponds are sometimes preferred.

The reaction takes place in tanks equipped with mechanical surface aerators or turbine aerators driven by the motors. In diffused air aeration the diffusers are placed along one side of the tank. This keeps the solids in suspension and permits oxygen absorption from the atmosphere. The diffusers may be of porous ceramic material, stainless steel, or perforated pipes etc.. Blowers may be centrifugal or positive displacement designed to operate at heads 8 to 10 psi, though it is affected by the depth of the tank, usually 10 to 20 ft. tanks may be as wide as 30 ft or more. When fine bubbles diffuser are employed, air should be filtered.

There are several types of aerators, the surface, the draft tube with the turbine, and turbine with the air injection being most common. The surface aerators is a paddle wheel or brush which is vertically suspended and partially submerged. The draft tube type is vertically suspended propeller, usually mounted in a vertical tube. The waste is pulled up or down in tube , effecting mixing and aeration. A modification consist of the vertically suspended turbine equipped with the sparge ring for dispersing the diffused air. Surface aerators may be up draft or the down draft. Mechanical aerators are rated on the basis of the oxygen input per horse power-hour, ranging from 3.5 to 4.5 for aerators varying from 1 to 150 horse power.

The micro organism culture is suspended in the waste, and since it is recycled, a final settling has to be provided. There is optimum solid content to be maintained in aeration tank, and consequently the rate of return of sludge from final settling tank is controlled.

There are three modifications of activated sludge process, generally categorized as
§ Conventional
§ Extended Aeration
§ Contact Stabilization
In the conventional process the aeration period ranges from 2 to 6 hrs. In extended aeration, it is upto 24 hrs. In contact stabilization process, the sludge in aerated for an extended period of time before it is mixed with the incoming waste. The main waste stream is aerated in the presence of sludge about 30 min. The process requires an aeration tank for the main stream, a sludge aeration tank, and a final settling tank.

Conventional activated sludge and contact stabilization processes are generally applied to clarified wastes and extended aeration process to raw waste. The extended aeration process produces very little sludge, and that produced is organically stable. It is therefore advantageous over two other methods, which dispose off appreciable quantities of excess, unstable sludge.

The design criteria include relations between BOD loading, tank volume, and aeration provided. For conventional activated sludge, the loading ranges from 25 to 300 lb. BOD /day (1000 cu feet). Diffused air requirements are 900 to 1100 cu feet/lb. BOD. For contact stabilization, the air requirement is 1600 to 1700 cu feet/lb. BOD. The sludge aeration in contact stabilization should have the holding capacity 2.5 hrs based on average flow and mixing or main stream aeration tank 30 min. In extended aeration plants the air requirement is 1500 to 3500 cu feet/lb. BOD. Loading varies from 12.5 to 25 lb. BOD/1000 cu feet.

Ion exchange: this is useful in the recovery of process chemicals as well as removal of objectionable constituents. The influent must be solid free. It is not competitive with other methods for de mineralizing brines.

The mechanism is chemical using resins that react with cations and anions. Two resins are employed which may be kept in separate receptacles or in the same mixed bed. The receptacles is usually of steel, totally enclosed and operating under pressure. Water is passed through it as an filter. The resin becomes exhausted after the period of time and must be regenerated with the acid and the anion type with caustic. The bed are operated at 6 to8 gpm/sq. ft of filter surface area.

The quantity of minerals that can be removed without the regeneration is dependent on the resin capacity and the concentration of the influent. The units have to be controlled to be assured that the resin capacity has not been exceeded. This can be done automatically by employing a conductivity cell in the control circuit. Provisions must be made for the disposal of the products of regeneration, which will be more concentrated than the original waste. Rinse water may also offer a disposal problem.

Chemical treatment Many waste are too acid or too alkaline to be discharged directly in the main stream and thus neutralization is necessary. Others, such as chromium plating waste, have constituents which may be removed by the reaction with the specific precipitants.

Neutralization or pH control requires a relatively small basin equipped with the means of agitation such as vertically suspended paddles or propellers. Air supplied by the compressors and dispersed through the diffusers is sometimes used. The design of mixing basin is covered under flocculation.

Acid, liquid caustic, or any solution may be proportioned into the waste stream by a diaphragm or other reciprocating pump. These are subjected to control by a variable speed motor or variable stroke length. The dry chemicals, such as lime, alum, and other coagulants, require gravimetric or volumetric feeders. These are usually equipped with a solution or slurry tank, so that the fluidity is achieved. The gravimetric feeders involve a weighing device, by means of which chemicals are added in proportions by weight. Volumetric feeders measure chemical addition by means of bulk of material. The former is, of course, the more desirable for precision in treatment.

Chlorination most industrial waste stream do not carry pathogenic microorganisms. However, chlorination of such waste waters is widely practiced for other reasons- including order abatement, BOD reduction, control of slime organisms, colour reduction, and as a specific reactant.

Chlorine can be added in the elemental form, which is stored and shipped under pressure as a liquid in steel cylinders. It may be also fed as a hypo chloride solution, either made up as it is used with calcium hypochlorite and water or purchased as a sodium hypochlorite solution. A third form used is chlorine dioxide.

Elemental liquid chlorine is fed by means of a device which meters and controls the amount of chlorine discharged from cylinder as a gas. Vacuum type feed is preferred to direct pressure feed for safety reasons. In both, chlorine flow is regulated by adjusting pressure differential across a fixed orifice or a varying size of orifice. In vacuum feed, the vacuum is created by water injector, which in addition to drawing the chemical from the cylinders places it in solution. At 20 0 Celsius, one volume of water dissolves 2.3 volumes of chlorine gas.

The chemical is highly toxic, and since leaks may develop, the equipment should be housed in a closed-off separately ventilated room, preferably equipped with an alarm system. The room should be heated to a temperature of 700 F. Rate of withdrawal or evaporation of the liquid is limited by the temperature. The rate may be increased by installing special heating devices.

Calcium hypochlorite is obtainable in the powdered, granular and tablet form and contains about 70 % of chlorine. It is batch dissolved in the solution tanks, usually of plastic. Sodium hypochlorite may be purchased in concentration of about 15 % available chlorine and may be fed in concentration directly. Both types require diaphragm or positive displacement type pumps for metering the solution into waste stream.

Chlorine dioxide, prepared from sodium chloride and chlorine, is a gas fed by metering equipment of water injection type. Stable chlorine dioxide solution may be handled directly without dilution by chemical proportioning pumps.