Treatment of Waste Water

This article throws light upon the nineteen traditional waste water treatments. They are: 1. Oil and Grease Removal 2. Aeration Type Units 3. Pressure Type Units 4. Vacuum Type Units 5. Skimming Devices 6. Square Settling Tanks 7. Circular Settling Tanks 8. Air Skimmers 9. Scum Disposal and 10. Soluble Organics Removal.

Treatment # 1. Oil and Grease Removal:

The terms ‘grease’ and ‘oil’ as used in waste water treatment denote a variety of materials, including fats, waxes, free fatty acids, calcium and magnesium soaps, mineral oil and other non-volatile materials that are soluble in water, and can be extracted by hexane from an acidified sample.

In average domestic waste, grease constitutes 10% of the total organic matter. Dairy, laundry, garage-machine shop and oil refinery wastes also have high grease-oil content.

It is usually required to reduce the grease content of the industrial wastes below 100mg/litre before it is discharged to a municipal system. Under quiescent condition, some portion of the grease settles with the sludge and some floats to the surface, i.e. as scum, where it may be removed by skimming.

(a) Grease Traps and Grease Interceptors:

These are used to collect grease and floating material from households, garages, restaurants, small hotels, hospitals, and the like. Owing to the danger of explosion or fire, motor oil, gasoline and similar light mineral oils should not be allowed to enter sewer systems.

Where the wastewater contains large amount of greasy kitchen waste, grease traps should be used. Commercial grease interceptors are also available for restaurants and smaller industrial establishments which are connected to public sewer systems.

(b) Grease Removal by Air-Aided Flotation (Flotation Units):

Grease and finely divided suspended solids may be converted to floating matter by air or gas-aided flotation units with or without flotation aids.

Gas-particle Contact:

The particles must be contacted by gas before floatation of the suspended solid is accomplished

Treatment # 2. Aeration Type Units:

i. Aerated Skimming Tanks, Pre-aeration: (Aeration before primary treatment):

This treatment also facilitates sedimentation and helps to refresh septic waste which—in combination with increased floating and suspended solid removal—improves the BOD reduction. For grease removal, 5 to 15 minutes’ aeration, using 0.01 to 0.1 ft³ per gallon (0.075 to 0.75 m³ per 100 liters) of air is usually sufficient.

ii. Manufactured Skimming Tanks:

Pressure and vacuum flotation techniques generate air bubbles by reducing the pressure of supersaturated air waste mixture or by applying vacuum to the mixture. The liberated minute bubbles tend to form around and attach themselves to suspended particles in the waste.

With this type of equipment, capital and operating costs are both relatively high. Therefore, this type of flotation unit is most popular for pretreatment of industrial wastes at the source where the flow rate is less and the concentration is high.

Treatment # 3. Pressure Type Units:

This process consists of pressurizing the wastewater with air, usually in a separate air-saturation tank, to 1-3 atm. and venting the tank to the atmosphere. This type of flotation involves high operating power costs.

Treatment # 4. Vacuum Type Units:

This process consists of saturating the waste water with air by aerating in a tank or by permitting air to enter on the suction side of the waste transfer feed pump. Under vacuum the solubility of air in the waste is decreased and air is released in minute bubbles.

The rising air bubbles attach themselves to the solid particles in the waste, carrying them to the surface. Because of the fairly high vacuum level involved, this type of flotation unit requires an expensive, air-tight construction.

Treatment # 5. Skimming Devices:

Scum collecting and removal facilities, including baffling are desirable ahead of the outlet weirs, on all settling tanks. Horizontal spraying with water under pressure may be employed to collect the scum for hand removal if no mechanical skimmers are installed.

Treatment # 6. Square Settling Tanks:

The straight scum baffle is installed ahead of the effluent weir-troughs and is submerged to a minimum depth of 18 inches (45 cm). Scum collectors usually move the scum towards the effluent side of the tank. Treatment plants are often equipped with hand-operated revolving slotted pipe skimmers installed horizontally across the tank ahead of the scum baffle.

If the scum is to be removed mechanically, cross-collectors consisting of endless chains above the surface are used.

Treatment # 7. Circular Settling Tanks:

The circular scum baffle is installed ahead of the circular effluent weir-trough and is submerged to a depth of at least 18 inches (45 cm). Scum removal is usually performed by a radial arm skimmer, which is attached to and rotates with the sludge-removal equipment.

Treatment # 8. Air Skimmers:

They may be used to serve small treatment plants. They work on the same principle as airlifts. They are provided with a funnel type extension to collect the scum.

Treatment # 9. Scum Disposal:

The scum is generally collected in a separate sump, which can be provided with dewatering facilities.

The scum is usually combined with the primary sludge and is disposed of by:

(1) Digestion, yielding gas with high fuel value;

(2) By vacuum filtration or incineration; and

(3) By Burial.

Treatment # 10. Soluble Organics Removal:

Soluble organics are very harmful from the physicochemical characteristic point of view for water as they directly affect other properties. Therefore, their removal is of utmost consideration.

Treatment # 11. Treatment Characteristics:

1. Septic Tanks:

Septic tanks received raw sewage, allow it to settle and pass the relatively clear liquid on to the adsorption field which is the next stage of treatment. The remaining solids digest slowly in the bottom of the tank. They are inexpensive but, because of their incomplete treatment, they are suitable only for small flows.

The anaerobic decomposition, which takes place in the absence of free 0₂ in the septic tank, is a slow process. In order to maintain practical detention times the reactions cannot be carried far. Therefore, the effluent is often malodorous; containing a multitude of microorganisms and organic materials, it requires further decomposition. Efficiency of this method is 15 to 25% reduction of BOD and removal of 40 to 60% suspended solids.

2. Imhoff Tanks:

The process that takes place in this tank is similar to septic tank, except that the tank is so designed that the flow through the upper chamber is separated from the lower digestion chamber, resulting in a two-storey tank. The upper compartment acts only as a settling zone where little or no decomposition takes place. This chamber often remains aerobic, and its effluent has a lower BOD than the effluent from a septic tank.

In the lower chamber, anaerobic digestion takes place. Since the effluent is of a higher quality, the process is suitable for communities and small cities. Additional treatment for further decomposition of the organic matter in the effluent is required. The efficiency of Imhoff tank is 25 to 35% reduction in BOD and 40 to 60% removal of suspended solids.

Treatment # 12. Suspended Solids Removal (Settling or Clarification):

Suspended solids removal from wastewater is traditionally by settling tanks or clarifiers—either longitudinal or circular. In either case, the design for domestic wastes is often based on overflow rate, a term relating to the flow per surface area which is equivalent to the settling velocity of the critical particle. The removal of suspended solids (in wastewater treatment) is almost always accomplished by gravity settling.

These units are known as settling tanks, clarifiers or sedimentation tanks. Removal of the solid matter from the liquid is accomplished by allowing the (heavier) solids to sink to the bottom of the tank while the clear liquid flows over weirs.

If the size of the solid particles does not change and if they are not influenced by their neighbours then the process can be called discrete particle settling. However, in most wastewater applications, the solids do undergo some flocculation with their neighbours and the process must thus be classified as flocculent settling.

An ideal settling tank is shown in (Fig. 8.1). The basin is divided into four zones: the inlet, outlet, sludge and settling zones. In the inlet zone, the incoming flow is assumed to be evenly distributed so as to afford uniform horizontal flow through the settling zone.

The outlet zone is used to collect the effluent for discharge over the weir. It is assumed that no settling occurs in these two zones. The sludge zone is provided for the collection and storage of the solids. The settling zone is where the actual gravitational separation is assumed to occur.

Treatment # 13. Types of Settling Tanks:

The most common type of device is the circular clarifier. This clarifier is relatively inexpensive to install and requires low maintenance. The central pier holds the sludge scraping mechanism which pushes the sludge to a central well from which it is pumped or drawn out by gravity.

The rectangular tank has the disadvantage of requiring submerged gears and chains, and these need more maintenance. It has often been suggested, however, that the rectangular tank has superior settling characteristics for similar overflow rates.

One design that eliminates the problem of submerged moving parts involves the use of a trolley with a submerged squeegee which moves back and forth, thus scraping the sludge to the sludge well. Settling tanks provide for the removal of material, which floats as well as sinks. Primary clarifiers are all equipped with scum removal mechanism which forces the solids to a separate well.

Tube Settler:

Its basic aim is to provide a great increase in the surface area and thus increase the efficiency. Tube settlers have been used successfully in water treatment plants for alum sludge and in final clarifiers for attaining greater suspended solids removal.

Treatment # 14. Ocean Dumping:

The concept of an infinite ocean (a mile and a half 2.4 km deep on the average around the whole world) has given way to the concept of the ocean as a limited and valuable resource. The resource must be protected—otherwise it may go the way of Lake Erie and the Baltic Sea, and the irreversible oceanic life systems may create an uninhabitable environment for people as well as for marine life.

Methods of Disposal:

The most common method of ocean disposal is to thicken the waste to a sludge or solid and submerge it to the point of disposal. When the waste is toxic it is usually put in containers and dropped in more remote places. There are so called approved areas on the East gulf and Atlantic coasts for waste disposal.

The sludge can also be disposed-off at sea through a pipeline either by diluting the digested sludge with the treated effluent from the plant or by reducing the solids content and allowing the solids to be diffused into the ocean together will the sewage.

The advantage in removing the solids and digesting them in advance of disposal is that this treatment results in as much as 80% reduction of volatile solids and as much as 99% removal of coliform and pathogenic bacteria. It appears that a low-cost alternative to ocean disposal is the disposal of digested sludge on croplands.

Ecology:

The environment of the sea is changed by the addition of wastes. Some authorities argue that organic compounds and nutrients benefit the sea and cite statistics on increased yield of fish per acre per year. The classical concept is that as long as oxygen is not depleted to the point at which fish are killed, wastes can be safely disposed-off in the sea.

Neither the level of dissolved O₂ nor the waste assimilating capacity of a body of water can be a safe criterion because both have serious limitations in a changing environment. In addition to the quantity of wastes there is a question as to the synergistic effects of many chemicals (synthetic) being discharged into the ocean.

Abstracts appearing regularly in the journals of water pollution control give a vivid picture of the need for stricter regulations for ocean dumping as well as in pollution abatement on a world scale.

Treatment # 15. Conventional Sewage Treatment Plants:

Conventional sewage treatment plants can probably be best identified by what they do not achieve, namely nutrient removal, demineralisation and removal of trace organics. Therefore, the conventional plant’s performance is usually measured by reductions in suspended matter, biochemical oxygen demand (BOD), and bacteria.

Conventional Sewage Treatment Processes:

Processes found in conventional plants include:

(1) Pretreatment

(2) Settling

(3) Chemical treatment

(4) Biological Oxidation

(5) Disinfection

(6) Sludge conditioning and disposal processes. Selection of Specific Processes

Selection of specific processes and plant design must take into consideration the existing and potential regulatory standards; plant operators’ requirements and availability; existing and projected sewage flow; the flow pattern and waste characteristics; climate, topography and availability of land; plant location within community; and all aspects of cost. Land availability and wise plant layout are two basic considerations.

Pre-Treatment Activities:

To protect pumping equipment, control and monitor instruments, and to avoid clogging filters, chemical feeders, valves and overflow devices, physical pretreatment processes are routinely included in all plants.

Pre-treatment equipment includes screens, grinders, skimmers and grit chambers. Flow equalisation is also considered to be a pretreatment process. Equalisation can assist in controlling hydraulic overloads that may occur during some period of the day and can also balance the incoming waste strength.

Precipitation (Settling):

Settling processes are used to remove settable solids by gravity settling either prior to or after biological or chemical treatment or in-between multiple-stage biological or chemical treatment steps. The solids are accumulated at an underflow withdrawal point by mechanical scrapers in larger tanks, whereas smaller and some older systems have a hopper bottom for solids collection.

Solids move down the sloped tank bottom by gravity in hopper bottom tanks. Both circular and rectangular tank shapes are used. A rectangular or square tank makes more efficient use of land area and can save construction costs by nesting units and using common walls.

Settling tanks are commonly designed on overflow rate, the unit volume of flow per unit of time divided by the unit of tank area (gallons per day per square foot).

Chemical Treatment:

Traditional chemical precipitation uses either iron or aluminium salts to form a floe which is then settled. Lime also clarifies. Suspended solids reduction up to 85% can be accomplished in this process. The accumulated chemical sludge is removed by gravity flow or pumping to conditioning or disposal or both.

The chemicals and sewage are flash-mixed in a mixing tank that has only a few minutes’ detention time followed by 30 to 90 minutes’ detention in a flocculation tank, which is slowly agitated to aid in the growth of floe. Thus, settling follows the flocculation tank.

Biological Oxidation:

Two basic techniques the fixed bed and fluid bed are utilised in conventional biological treatment. The trickling filter employs a fixed bed of stone or plastic packing materials to provide a surface for the growth of the zooglea, bacteria and other organisms. The intermittent sand filter and the spray irrigation systems are other examples of the fixed-bed technique.

The various activated sludge processes and sewage lagoons are fluid-bed systems. The activated sludge process uses mechanical aeration and return of a percentage of the active sludge to the process of influent. Lagoons or stabilization ponds and the oxidation ditch do not routinely waste sludge, but multi-pond systems may have recirculation.

Septic tanks and Imhoff tanks combine, in a single tank, for the settling and biological oxidation processes.

Treatment # 16. Activated Sludge:

Processes that use continuous agitation and artificially supplied aeration of settled sewage, together with recirculation of a portion of the active sludge which settles in a separate clarifier back to the aeration tanks, are classified as activated sludge processes. There are variations as to detention time, method of mixing and aeration and the technique of introducing the waste and re-circulated sludge into the aeration tank.

Treatment # 17. Trickling Filters:

These are the most common treatment units used by municipalities to provide aerobic biological treatment. They are classified by the hydraulic and organic loading applied. Sludge from the final clarifier is usually re-circulated to a point ahead of the primary settling tank.

Single-stage and multistage filter arrangements are both utilised because there are many recirculation schemes and many options of intermediate settling between multistage filters. All returned sludge from the intermediate and final clarifiers that is not wasted (excess) is returned ahead of the primary settling tanks.

Treatment # 18. Lagoon and Oxidation Ponds:

The lagoon refers to a pond of engineering design that receives waste which has not been settled or exposed to biological oxidation prior to entering the lagoon. Simplicity is the main feature of the raw sewage lagoon. The oxidation pond, as opposed to the raw sewage lagoon, receives an influent that has undergone primary treatment.

The maturation pond provides a final, polishing treatment step that follows some form of secondary treatment. The oxidation pond may be mechanically aerated to improve treatment and reduce pond size. There are many applications for lagoons and ponds, ranging from complete treatment of raw waste to polishing of secondary plant’s effluents.

Treatment # 19. Septic Tank:

It continues to serve as the wastewater disposal system for millions of household and for numerous small industries. The system combines settling and anaerobic surface disposal, usually by open jointed tile under drain network. Since the septic tank almost always operates without power, an automatic siphon is used for dosing by discharging the chamber contents each time the level reaches a fixed point.

Imhoff Tank:

A two storey tank uses the upper chamber for settling and the lower chamber for sludge digestion. It can be followed by additional process steps to improve the quality of the plant effluent. The intermittent sand filter furnishes additional treatment to an Imhoff tank effluent.

Selection of the sand filters to be dosed is usually a manual operation controlled by valves. 3 to 4 inches (7.5-10 cm) of sand is normally laid over about 6 to 12 inches (15-30 cm) of gravel. Tile under drains collect the effluent, which is discharged to the receiving waters. Uniform distribution of flow to the sand filters is important and is usually achieved by distribution or by a rotary arm distribution device.

Disinfection:

Conventional treatment plants use chlorination as the final treatment process to reduce the concentration of bacteria. Pre-chlorination, performed on the plant influent, is used if the incoming sewage is septic or if the flows are low and the holdup time in the plant is such that the waste will become septic. Pre-chlorination is usually at a fixed dosage, and a residual chlorine level is not maintained.

For post-chlorination 15 to 30 minutes detention time in a baffled, closed tank to prevent short circuiting and dissipation of the chorine is provided. Chlorination is not frequently used to reduce the BOD in the plant effluent. Ordinarily, a combined chlorine residue between 0.2 and 1 mg per litre is the target for the final effluent.

Sludge Treatment:

The settled solids material from the various treatment processes is called sludge. For the excess quantities of sludge to be disposed-off economically and with a minimum of objection, the raw sludge is digested and dewatered. Controlled digestion reduces the quantity of complex organic materials, increases the number of ultimate sludge disposal alternatives and decreases the undesirability of the sludge.

Sludge Storage:

Excess quantities of activated sludge and sediments from primary, intermediate and final clarifiers are routed through the steps of mixing, digestion, dewatering and/or conditioning. Some form of short term storage is essential to prevent overloading, regulate sludge flow to the digesters and allow collection and mixing of the sludge.

Some plants mix in some final effluent with the sludge (in the mixing tank) to improve its thickening characteristics. Storage tank should be open to avoid gas buildup in the tank.

Sludge Thickening:

Gravity thickening in a deep circular open tank is frequently used ahead of anaerobic digestion; polyelectrolyte has also been used but their use is not widespread. Air floatation thickening of activated sludge is an alternative.

Dissolved or diffused air and, in some cases, a coagulant are used to float the sludge to the surface where it is removed by a mechanically driven skimmer. The effluent draw-off near the tank bottom is combined with incoming plant influent and passes through the treatment cycle.

Sludge Digesters:

Anaerobic digestion is more common than aerobic digestion because the anaerobic process does not require an air supply and generally produces enough gas to provide digester with fuel for heating. It can be classified as low rate, high rate and secondary units. The low-rate system merely holds the sludge for long periods (30 days or more).

The high-rate system employs some form of mixing and heating to accelerate digestion and, consequently, has shorter detention periods (10 to 20 days) and can accommodate increased solid loadings. The high rate unit is typically followed by a larger secondary digester that furnishes stratification of supernatant and sludge.

Aerobic sludge digestion requires an extensive air supply to maintain a dissolved oxygen surplus in the digester. Aerobic tanks have the advantage of not being subject to upsets and of producing a more treatable supernatant but the costs of air make the operating cost of aerobic digestion higher than that of anaerobic digestion.

Sludge Conditioning:

Eluciration and chemical addition are the two sludge conditioning alternatives. Conditioning is an intermediate process between primary and secondary anaerobic digesters and also improves dewatering on digested sludge. Iron and aluminium salts and lime are the most popular chemicals for conditioning.

Eluciration is the washing of suspended sludge held in suspension by air or by stirring. It reduces the alkalinity and makes the sludge more filterable. Single and multi-tank (countercurrent) eluciration are both used. The single tank system uses repeated washing of the sludge, whereas in the countercurrent system the fresh wash water is added to the last sludge tank.

Sludge Dewatering:

Rotating drum vacuum filters serve as the conventional sludge dewatering equipment. The filtrate is returned to the plant influent and the sludge cake is disposed-off. Sludge drying beds are common in smaller plants for disposal. Weather conditions are an important controlling factor when open drying beds are used.

Drying beds have under-drain networks of tile fields laid in sand and gravel. The discharge from this network should be disinfected if it is not returned to the treatment plant. Sludge treatment and disposal are usually carried out in separate treatment plants.