Treatment & Utilization of Industrial Wastewater in Kuwait

In this article we will discuss about the treatment & utilization of industrial wastewater in Kuwait.

N. Al- Awadhi and K. Puskas

Kuwait Institute for Scientific Research P.O. Box 24885, 13109, Safat, Kuwait

Abstract:

The treatment and effective utilization of treated industrial wastewater effluents are required in order to protect the environment, to restore and maintain national water resources, and to ensure a cost-effective water supply for the agriculture and greenery sectors which are highly dependent upon water.

The paper describes treatment and the reuse options which are viable, in Aridland countries such as Kuwait, and recommends treatment criteria for the different uses. The paper also describes in detail the different treatment technologies and experiences that has been demonstrated in Kuwait for the treatment of industrial wastewater.

Introduction:

The treatment and effective utilization of treated industrial wastewater effluents are required in order to protect the environment, to restore and maintain national water resources, and to ensure a cost-effective water supply for those sectors highly dependent upon water.

The reuse of treated industrial wastewater should be a critical component of Ku­wait’s water management plan, such reuse planning should take into account the type, quantity, quality, and location of industrial effluents, and the availability of technically and economically feasible treatment and reuse technology.

The circumstances which have changed in postwar Kuwait have also influenced the available water resources, and the water allocation policies and strategies, and should be considered during the implementa­tion of any effluent treatment and reuse scenario. Technical, economic and environmental considerations, as well as health, safety and strategy will define the viable reuse options.

Treatment and Reuse Options:

Treatment technologies which can result in reusable effluents and meet economic requirements are preferred for handling all types of effluents.

The options for an on-site treatment plant for an industry, or a central or an area treatment facility to handle combined industrial effluents should be based on the results of technical and economic feasibility studies. The combination of on-site and central wastewater treatment can also be a feasible solution for economic treatment of the effluents for safe reuse.

Among the reuse options available, greenery, roadside plantation, and forestry irri­gation have top priority according to prior use practices for treated industrial effluent in Kuwait. Agricultural irrigation can be performed with treated industrial wastewater, e.g., from food processing and agricultural industries, where hazardous effects on the food chain are excluded.

In some cases, in-plant reuse of treated wastewater for industrial or irrigation pur­poses can be a viable reuse option. General in-plant usage is the use of treated wastewater for washing and cleaning. The building industry can use treated effluents, e.g., for ready- mix concrete and precast element production. The reuse of treated effluent is not eco­nomic if high quality water is required, e.g., boiler feed-water.

Internationally and locally developed technologies and systems adapted to local conditions are available for the treatment and reuse of most of the wastewater generated in Kuwait. Technologies have been developed to treat the petrochemical and organic wastewater that is generated at larger industries, and also for smaller wastewater genera­tors, such as car washing stations. The technology and equipment are available on the market to recycle such effluents.

The economic benefits, and therefore, the widespread implementation of this tech­nology is influenced by the presently existing subsidy system, which keeps the price of energy, fuel and water at a low level.

Therefore, wastewater generators have limited interest in the implementation of efficient water treatment and reuse systems. The system of control and enforcement, and the rules and regulations for wastewater management, have a significant influence on the implementation of industrial wastewater treatment and reuse technology.

Treated Effluent Quality Criteria:

Treated wastewater must meet quality criteria which depend on its intended disposal or reuse. The quality criteria for sea disposal are summarized in Table 1. As an example, the mass load limits are also given in the table. These mass loads were considered in the design of the Central Wastewater Treatment Plant (CWTP) for the Shuaiba Industrial Area (SIA).

Treated-effluent quality criteria for greening irrigation, according to Hamdan (1990), are given in Table 2 for the chemical and microbiological contaminants. The proposed limits for the micro-pollutants (heavy metals) are summa­rized in Table 3.

Treatment and Utilization of the Wastewater Generated from Industrial Areas:

i. Shuaiba Industrial Area (SIA):

The industry with the most significant quantity (30,000 m³/d) of wastewater is the petrochemical industry within the SIA. The SIA’s wastewater is the most valuable wastewater resource for reuse because of its significant quantity and its centralized location. There is a good possibility of obtaining a high quality effluent.

ii. Centralized Treatment Facility:

The location of the petrochemical industries and the refineries in a single industrial area offers great opportunity for a technically and eco­nomically feasible central treatment plant. The treatment technologies for the reuse of the SIA’s wastewater at a CWTP have been investigated and are well known.

The reuse of the treated effluent for greenery irrigation was the most feasible reuse option. The centralized treatment of the SI A’s combined industrial and sanitary wastewater has many advantages, and results in a more controlled and economic treatment.

As prior studies have revealed, the combined flow can be treated efficiently at a central facility whereby pH of wastewater will be neutralised by the mixing effect of the inflow of acidic and basic waste streams, and the temperature can be balanced also to obtain an adequate temperature for the second-stage biological treatment. The necessary nutrients for the biological process can be provided from the nitrogen-rich sanitary wastewater effluents.

Better management and control can be introduced for the plants operation and effluent quality. According to well-known economics laws, the establishment and opera­tion of a greater size plant is more economic. Centralized treatment offers easier reuse, better water allocation strategies, and more feasible options for effluent upgrading, if needed for special reuse options.

Centralized treatment will not exclude the need for on-site treatment of effluents, which can be toxic or highly polluted with compounds, more easily removed from a stream with a lower flow rate. For example, such effluents are the Petrochemical Industries Company (PIC’s) urea and ammonia plant effluents, which are hydrolysed before they are discharged from the plant into the neutralizer pit.

According to the results of one study, the advantages of a CWTP implemented for the SIA’s combined wastewater, can compensate for all of its dis-advantages, such as the need for an industrial wastewater drainage system, wastewater manage­ment, and waste-handling strategy differences among the SIA’s industries.

iii. The Central Wastewater Treatment Plant (CWTP):

A schematic diagram of the proposed CWTP is shown in Fig. 1 and its layout on Fig. 2. The preliminary treatment is followed by inter mediate, secondary and tertiary treatment steps. The excess biomass, i.e., sludge, is treated by a separate sludge treatment system. Both the industrial and combined wastewater can be treated at such a treatment plant.

By means of screening and grit removal, organic and inorganic coarse materials, e.g., stones, fibres, paper, and plastic can be removed. Oil removal by an API separator is designed to pre-treat oily material generated from the refineries. Therefore, the loading of the bioprocess can be reduced.

In the balancing tank, the temperatures and pH of the various streams can be neutralized and equalized. Further pH adjustment, i.e., control, is possible by adding chemicals in the pH control basin. As per the results of a survey of the effluents, hot flows might enter the plant, which cannot be ballasted.

A cooling unit is designed in the system to reduce the temperature of the hot wastewater streams to a level acceptable for the biological process. In the intermediate treatment stage, a second oil removal by induced air floatation (IAF) provides further reduction of the oil.

Biological treatment, the activation sludge process, removes the dissolved biode­gradable organic materials, i.e., reduces the main portion of the Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) of the combined wastewater. The final setting of the sludge, which is the excess biomass, is a part of the secondary treatment.

The tertiary treatment provides further reduction of the remaining, mainly suspended, pollutants in the wastewater by sand filtration. Disinfection is performed by chlorination.

This series of unit operations designed to treat the SIA’s industrial wastewater can result in an effluent quality adequate for greenery irrigation. A common system, which consists of a sludge storage /blending and dewatering unit, would serve for the treatment of the sludge. The treated sludge would be disposed of at the solid waste treatment plant for use in the landfill pit.

The layout of the proposed treatment plant shows the comparative size of the units. A larger area is required for the balancing and emergency storage basins. Two parallel biological systems (No. 9, the activated sludge basin, and No. 10, the final setting) provide safe operation for the sensitive bioprocesses. Taking into consideration the development of the industrial area, phase-by-phase implementation at the same plant location has economic benefits.

iv. Independent Treatment and Reuse of Special Effluents:

Some effluents in this area require special attention in order for some of their pollutants to be utilized cost-effectively via specially developed technologies. One example of such an effluent is the nitrogen-rich wastewater generated from the PIC’s fertilizer division.

This effluent can be converted into soil stabilizer or slow-release fertilizer by chemical treatment. A study has revealed its technical and economic feasibility.

v. On-Site Treatment of the Refinery Wastewater:

Centralized treatment might not be implemented because of the differences in the waste management policies of the indus­tries, or for some other conditions (most of which are financial). In this case, the industrial effluent should be treated through an on-site treatment facility. The three refineries in the SIA, and PIC already have such wastewater treatment facilities.

The complexity of a treatment system varies with the complexity of the industry. A simple case would be a treatment facility for a refinery having a crude oil unit with a desalter, a small reformer, and a desulfurization unit. In as much as the storm-water retention pond stabilizes the flow to the separator, it also serves as a source of make-up water for the cooling tower and boiler.

The API separator and the dissolved-air flotation unit remove free oil and suspended solids. Effluent from the dissolved-air flota­tion is treated in an aerated lagoon to reduce dissolved organic materials. The final effluent is then filtered for suspended solids removal.

A refinery having cracking operations is the next order of complexity. The major change in the wastewater treatment technology, is that the aerated lagoon be replaced by the activated-sludge system. An aerated lagoon can be modified to operate as an acti­vated-sludge system by adding a sedimentation tank and sludge-recycling pumps. This might be a consideration for a refinery having an aerated lagoon, and needing to increase the efficiency its of organic material removal.

Another step in improving effluent quality might be the addition of carbon-adsorp­tion facilities. This is usually justified only in cases where an effluent of the highest quality is required, or if total recycling and reuse of wastewater is being practiced.

Any design of wastewater treatment facilities has to consider water reuse or the zero discharge of pollutants. Although each wastewater treatment problem is unique, similar cases can be compared and ideas obtained as to which treatment process can be used to solve a particular problem. In many instances, it may be necessary to set up a pilot plant to determine which process can best do the job.

Recommendation for Reuse:

A recommendation has been prepared for the utilization of the treated effluent from the CWTP. Many options, e.g., irrigation, industrial reuse, groundwater recharge, and beautification of the area by creat­ing multipurpose lakes, were studied. The greening alongside the Nuwaiseeb Highway was found to be the most feasible option, in agreement meet with the water allocation strategies.

vi. New Mina Abdullah Industrial Area (NMAIA):

The effluents generated from the New Mina Abdullah Industrial Area’s (NMAIA’s) inorganic industries have less value for recycling, since they constitutes relatively small amount (i.e., 2-3,000 m³/d). After simple on-site treatment (e.g., settling and pH adjustment), some of the effluents can be used for greenery irrigation in or around the plants, or they can be directly recycled, e.g., for ready- mix concrete production. The effluents could be treated at the CWTP, a scenario which can be feasible, if the CWTP’s location is accessible to the NMAIA.

Other Industrial Areas and Individual Industries:

Wastewater from other indus­trial areas as well as governmental institutions, e.g., army bases, produce typical effluents which can treated on-site treatment by means of individually designed treatment facili­ties, package plants, or specific systems developed for local conditions in Kuwait (e.g., high-rate algal ponds for treatment of wastewater effluents with a high organic content).

Significant amounts of food, food processing and agricultural industries generate wastewater with organic contents, which can be treated by biological methods to obtain treated effluent suitable for irrigation.

i. Sabhan Industrial Area (SBIA):

Soft drinks, organic and inorganic industries dis­pose of ~ 4000-5000 m³/d of wastewater into the municipal sewer. The most significant industrial area, after the SIA, is the Sabhan area, where the wastewater generation is concentrated in a relatively small area.

Therefore, collection and treatment can be carried out in a combined manner, and the treated effluent can be reused in the area for greenery and roadside irrigation. The wastewater effluents, especially from the bigger generators, e.g., refreshment industries, are valuable water sources for irrigation after on- site treatment.

ii. Shuwaikh Industrial Area (SHIA):

Various qualities and quantities of wastewater are generated from the various industries in the Shuwaikh Industrial Area (SIA). The industrial wastewater, combined with the municipal wastewater, is treated at the Ardiya Wastewater Treatment Plant. Industrial wastewater makes up 20-30 per cent of the total flow rate.

Effluents from mechanical workshops, car service stations, slaughterhouses, and food processing plants are the typical wastewater effluents to be considered for the characterization of the treatment technologies in this area. Presently, the on-site treatment of the industrial effluents is the only option, since separated drainage does not exist in the area for the industrial wastewater.

Wastewater Treatment Processes:

Taking into consideration the government’s policy for wastewater management, scientists and engineers can decide which treatment processes, or combination of proc­esses, will best perform the necessary job of cleaning-up the wastewater effluent in­volved. Since waste-treatment facilities contribute significantly to the total cost of a project, and the cost involves also land and manpower, future needs must also be consid­ered.

This is often difficult to do, because regulations change or new regulations come into effect. For this reason, a development program should be instituted, so that as wastewater quality requirements become more stringent, treatment processes can be modified, or new ones can be added to improve the water quality. As a plant expands, a similar program may be followed to increase its wastewater handling capacity.

An effluent-treatment system involves one or more stages, depending on the quality of the raw effluent and the required pollutant reduction. The various unit processes to be used for the treatment of the industrial wastewater effluents in Kuwait, to yield the quality acceptable for the selected reuse are categorized in Table 4. They can be used in various combinations or as single technologies, depending on the type of the wastewater and the quality requirements for the effluent’s reuse or disposal.

i. Primary and Intermediate Treatment:

The primary and intermediate treatment is performed by chemical and/or mechanical processes to prepare the wastewater for the secondary, biological treatment, or to prepare the effluent directly for disposal reuse. Grit removal and settling are generally used for municipal and various industrial wastewater effluents (e.g., organic, inorganic and petrochemical wastewaters).

pH control and neu­tralization are used in the refineries, and e.g., the fertilizer industries (PIC’s fertilizer division), for pre-treatment of non-neutral effluents. Sulphide/ammonia stripping is a typical process for refinery wastewater treatment to reduce the amount of H₂S, NH₃ and other chemicals (e.g., phenol).

Chemical treatment, liquid-liquid extraction and filtration for oil removal are applied in petrochemical and other industries to reduce the amount of the chemical compounds or convert them to valuable compounds, which can be used (e.g., nitrogenous compounds in PIC’s nitrogen-rich effluents are converted to fertilizer and soil stabilizer).

If the effluent quality goals cannot be reached by the primary treatment, intermedi­ate treatment can follow it, to upgrade and produce a final effluent or provide an accept­able quality effluent for the secondary treatment.

Sulphide/Ammonia Stripping, Oil Removal:

Stripping of sour-water streams gener­ated in the Shuaiba Refinery was investigated by Kuwait Institute for Scientific Research (KISR). Significant amounts of H₂S and NH₃ were found in refinery wastewaters due to the breakdown of organic sulphur and nitrogen compounds during the various refining processes. These compounds can be removed by air or steam stripping.

A pilot-plant-size packed-glass stripper was constructed in KISR’s laboratory. Effi­cient stripping was obtained using 0.3-0.6 kg steam/1 kg feed. Removal efficiency was 99 per cent for H₂S, and 75 per cent for NH₃. The stripped water is normally utilized as feed-water for the crude-oil desalters.

The SIA’s refineries are large enough to justify recovery of the H₂S and NH₃ as products, to help reduce the capital and operating costs of the process.

ii. Chemical Treatment:

A special utilization of the chemical treatment was studied for use with PIC’s nitrogen-rich effluents. The chemical treatment of nitrogenous effluents is aimed at eliminating an environmental problem, and producing an effluent that can be used for irrigation, and due to its urea-formaldehyde (UF) content can also be used as a binding material to stabilize sand and soil.

Studies of the wastewater effluents generated in PIC’s fertilizer division showed that, in terms of treatment, the effluents can be divided into two groups: nitrogen-rich streams with 1-1.5 per cent urea and 0.13 per cent ammonia contents, and a 1900 m³/d flow rate; and nitrogen-poor streams with around 0.05 per cent urea and 0.07 per cent ammonia content, that are also contaminated by other pollutants, and a 2000 m³/d flow rate.

The Formaldehyde Acidic process (FA) was selected to convert these nitrogenous compounds in low concentrations to polymers that stabilize and condition soil because the polymer formation is enhanced by acidic catalization. The batch and continuous proc­esses are demonstrated in Fig. 4.

The mixed monomers are produced in the first reaction stage reactor (R₁) when formaldehyde is added to the wastewater in an alkaline or neutral condition. The second-stage reaction starts when the liquid is acidified. Water-soluble/ insoluble resins are produced in this reaction stage (R₂ reactor).

The amount and type of resin depend on the reaction conditions. The urea concentration, UF ratio, acidification, and curing temperature are the most important factors. The diagram in Fig. 3 shows the efficiency of the polymer formation for a 4 per cent urea wastewater effluent.

The laboratory-scale experiments successfully produced soil stabilizers from the nitrogenous wastewater effluents. The nitrogen-rich effluents could be treated efficiently without adding urea; 30-60 per cent of the urea in the wastewater could be converted to the water- insoluble resin that provides the soil stabilization effect.

FA treatment for the combined (nitrogen-rich and nitrogen-poor) effluent without the addition of urea is technically feasible, but the efficiency is lower. Additional urea enhances the formation of the water- insoluble resin; thereby, increasing the efficiency of the process. If the urea concentration is increased to 4 per cent (Fig. 4), the maximum amount of urea can be converted to water-insoluble resin. The polymerization time can be controlled by pH adjustment.

An appropriate soil stabilization effect was reached on sandy and gatchy soil samples from Wafra, Sulaibiyah and Abdally by applying the wastewater that had been treated in the laboratory. A continuous surface layer was obtained by spraying the soil’s surface with the stabilizer, which has some capacity to resist weather conditions in Kuwait.

Useable liquid soil stabilizers can be produced from the wastewater by the FA process using an industrialized process (Fig. 5). Besides this principal purpose, the wastewater treatment problem can also be simultaneously solved. The process can be adjusted to get optimum soil stabilization for several uses. Approximately KD 2.5/m³ is the production cost of the liquid soil stabilizer, which is suggested for use in agriculture, landscaping, and environmental protection. A higher quality liquid soil stabilizer costs KD 7.5/m³.

If the demand for the liquid soil stabilizer does not persist, the FA process could be used as an easy and inexpensive way to treat the wastewater for disposal. The treatment cost in this case, would be about KD 0.7/m³.

Gravity Separators:

Wastewaters that contain a high amount of free oil are normally treated in the gravity separator before they are mixed with other, non-oily streams. This allows the separator to be sized for a smaller through-put volume. Sometimes the addition of chemicals improves the separation.

American Petroleum Institute Separator:

API separators are installed and efficiently operated in the Shuaiba refineries to reduce the oil content in the wastewater before any further treatment and disposal. The traditional method of oil separation in refinery wastewater is via the API gravity separator, which is sized to allow most of the free oil to float to the surface, and the heavier solids to fall to the bottom. These separators are normally an integral part of the operation, due to the amount of recoverable oil in refinery wastewaters.

Several parameters determine the effectiveness of a separator, among which are the temperature of the water, density and size of the oil droplets, and the type of solids in the water. However, only free oil is separated; emulsions are not broken down. Removal efficiencies of 60-99 per cent for oil and 10-50 per cent for suspended solids have been reported.

Tilted-Plate Separator:

This unit is made up of one or more modules consisting of several corrugated plates tilted at 45° angles. As the water flows between the plates, the oil droplets collect on the underside and move to the top of the module. Improved efficiencies at less cost and space than those needed by the API separators have been reported.

pH Control:

Since many waste streams are highly acidic or alkaline and would therefore be detrimental to biological processes it is necessary to control the pH of refinery wastewaters before they proceed to biological treatment. Sometimes, phosphoric acid and ammonia addition are beneficial for the dual purpose of controlling pH and supplying necessary nutrients for the downstream biological processes.

Dissolved Air Flotation:

Dissolved air flotation is used in the Shuaiba Refinery to produce an acceptable effluent quality before discharge. It is normally considered an intermediate or secondary treatment process for further removal of oil and suspended solids from pre-treated effluents, prior to biological treatment or disposal with cooling seawater dilution. The effluent from the dissolved air flotation unit contains 30 ppm oil before dilution.

Weisberg and Stockton (1973) report how the American oil refinery in Whiting, Indiana, involved flotation which included a series of bio-flotation ponds to treat effluent from secondary treatment. Apparently, chemical coagulation and air flotation of the effluent from the aerated lagoon produced a final effluent that met tertiary treatment goals.

For dissolved air flotation, removal efficiencies of 70-85 per cent for oil, 50-85 per cent for suspended solids, 20-70 per cent for BOD, and 10-60 per cent for COD, were measured and reported at the Shuaiba refinery.

Pilot-plant tests on flocculation-flotation have shown 97 per cent removal of oil, 75 per cent of solids, and 80 per cent of BOD and COD. The chemical flocculating agents used to aid flotation were activated silica, and aluminium and ferric salts. These agents also improved the effectiveness of the flotation unit in removing oil emulsions.

Since the effectiveness of dissolved air flotation units varies substantially with the characteristics of the particulate matter, it is imperative that laboratory and pilot tests be run to determine the necessary design criteria. In the dissolved-air flotation process, the waste stream is saturated with air under a pressure of several atmospheres.

This is done by pumping the stream to 40-50 psig, and feeding compressed air through the pump’s suction. The stream is then held in a retention tank for several minutes so that the air will dissolve. As the stream exits the retention tank, it flows through a pressure-reducing valve which drops the pressure to atmospheric and then into a flotation tank.

When the pressure is released, the air comes out of solution, forming minute bubbles within the liquid. These bubbles, which are about 30-120 µm in diameter, form on the surfaces of the suspended particles. An aggregate is thus formed, which rapidly rises to the surface and is skimmed from the flotation tank. Design details are provided in API (1986).

Packaged units are available from various vendors. Franzen (1982) reported the use of surplus boxes in the API separator as flocculation chambers, and indicated that coagulant concentration proved to be important. For instance, a concentration of 30-50 ppm of alum was sufficient to promote coagulation without deterioration of effluent quality due to excessive coagulant.

Equalization:

Wastewater (e.g., refinery) flow rates may vary significantly from time to time. It is very important for the effectiveness of biological processes that the volume and composition of the feed be fairly constant. Therefore, a holding point with sufficient residence time to even out major fluctuations is needed. This not only attenuates normal process variations but helps to minimize the shock of a major spill.

iii. Secondary Treatment:

Secondary treatment is a biological operation unit which is capable of removing the dissolved organic materials expressed BOD or COD. The listed technologies (Table 4) can be used in Kuwait for wastewaters with biodegradable pollutants.

A study was carried out treating the combined wastewater effluent from the SIA’s industries with activated sludge. The tricking filter is used in Kuwait at some of the army bases to treat combined sanitary and industrial types of wastewater. Waste stabilization ponds were thoroughly investigated to adapt this technology for the treat­ment and reuse of municipal and industrial effluents with high organic matters.

The following wastewater treatment processes are those that improve the quality of wastewater to the point where it can be reused within the refinery or for irrigation, or be discharged without any adverse environmental reaction. The extent to which the process is used depends on the quality of the raw effluent, the pollutants contained in the raw effluent, and the method of ultimate disposal or reuse of this water.

Activated Sludge Process:

An activated sludge process was designed for the CWTP in the SIA. Activated sludge procedures are used extensively for coagulating and removing non-settleable colloidal solids, as well as for stabilizing organic matter. The US EPA (1982) reports removal efficiencies of 80-90 per cent for BOD, 50-95 per cent for COD, 60-85 per cent for suspended solids, 80-99 per cent for oil, 95-99 per cent for phenol, 33-99 per cent for ammonia, and 97-100 per cent for sulphides using activated sludge. Thus, a high quality effluent is obtainable from a properly designed and operated activated sludge system.

The activated sludge process is an aerobic biological procedure containing, within a reaction tank, a high concentration of microorganisms, which is maintained by recycling the activated sludge material. Oxygen is supplied to the wastewater in the reaction tank either by mechanical aerators or a diffused air system. In as much as the microorganisms remove the organic materials by biochemical synthesis and oxidation reactions, the con­verted organic matter must be removed by sedimentation, prior to final discharge.

The main components of the process are the aeration, or reaction, vessel and the sedimentation tank. Sludge removed from the sedimentation tank is recycled to the aeration vessel to maintain the required concentration of microorganisms. Since a portion of the sludge must be discarded, it is first dewatered and then used as landfill.

Nutrients primarily nitrogen and phosphorus are required to maintain a healthy growth of microorganisms within the system. These elements are needed in minimum amounts of 5 kg nitrogen and 1 kg phosphorus /100 kg BOD to be removed. Generally, refinery wastewater contains enough ammonia to supply adequate nitrogen, but it may be deficient in phosphorus.

Some of the sources of these elements are:

1. Water condensate from catalytic cracking units which normally contains ammo­nia and other nitrogen compounds;

2. Spent phosphoric acid catalyst from polymerization units; the phosphates can be leached from the catalyst with water;

3. Boiler or cooling-tower blow-down, which contains phosphates; and

4. For the combined effluent in the SIA, the PIC’s nitrogen-rich wastewater from the fertilizer division.

Occasionally it may also be necessary to adjust the pH of the waste stream and add nutrients. Nitrogen and phosphorus in acid or alkaline compounds can achieve both of these purposes.

An inadequate supply of nutrients results in poor stabilization of the wastes and will stimulate fungi growth. This results in poor sludge-settling characteristics, and longer periods of aeration.

The oxygen required by the activated sludge system varies between 0.6 and 1.5 kg 0₂/kg BOD removal. For design purposes, an oxygen demand of about 1.0 kg O₂,/kg BOD is often used; however, this should be determined experimentally before designing a particular system. For the combined wastewater streams generated from the SIA’s refin­eries, 0.6-1.3 kg O₂/kg BOD was established.

Trickling Filters:

The trickling filter is an aerobic biological device that is used extensively in the refining industry. It may be used as a secondary treating system by itself, whenever a high-quality effluent is not required. This filter may also be used upstream of an activated sludge unit, to reduce the loading or to attenuate the organic loading on the unit.

A trickling-filter system consists of a filter bed with a wastewater distributor and a sedimentation tank. The filter is usually a bed of broken rock or coarse aggregate; recently, plastic sheets have come into use as filter media. Rocks ranging in size from 2- 4 in. are best, so as to get a maximum amount of surface while maintaining good ventilation through the bed. Beds range from 1-1.5 m deep, and can be either circular or rectangular.

If plastic media are used, they can be up to 10 m deep. Sprinklers are used to evenly distribute the wastewater being fed to the filter over the bed’s surface. A sedimen­tation tank is needed for clarification of the effluent, due to the biologic growth that sloughs from the filter bed.

The US EPA (1982) reports removal efficiencies of 60-85 per cent for BOD, 30-70 per cent for COD, 60-85 per cent for suspended solids, and 50-80 per cent for oil. The efficiency of a particular filter depends significantly on its loadings.

Wastewater Treatment Ponds:

Wastewater treatment ponds are effective and ben­eficial in Kuwait where the biological treatment system is not affected by cold weather conditions or ice on the pond surface. Areas are available to construct ponds with rela­tively large surfaces. Wastewater treatment ponds can be constructed with low capital, and operated with low running costs. Their operation and maintenance is relatively simple. Operator attention is not needed, because no mechanical equipment is involved.

Wastewater treatment ponds can be used for treatment of refinery wastewater, but in Kuwait, under the present conditions, when large refineries with high effluent flow rates are concentrated in a single industrial area, more compact treatment systems are required.

Their usage is preferred for industries generating organic wastes, such as food processing and agricultural industries, and farms, which are not yet connected to the sewer system in Kuwait. Wastewater treatment ponds can produce treated effluent, which satisfies the criteria for landscaping and agricultural irrigation. Ponds were operated with great success on Failaka Island for municipal wastewater treatment for many years before the war.

Waste-stabilization ponds, aerated lagoons, high rate algae ponds, and combined ponding systems can be used, depending on the wastewater’s characteristics, the opera­tional conditions, the requested effluent quality, and the intended reuse. It is difficult to develop any general design parameters for oxidation ponds.

Any parameter must be functionally related to temperature, solar intensity, wind, organic loading, and the nature of the waste. To provide design parameters for pond construction in Kuwait, high-rate algae ponds and their combination with facultative ponds for treatment of organic indus­trial and municipal wastewater were studied and adapted for local conditions.

Waste-Stabilization Ponds:

A stabilization pond is simply a large shallow pond in which bacteria stabilize the wastewater fed to the pond. Stabilization ponds are classified as aerobic, aerobic-anaerobic, and anaerobic, depending on the type of biological activity involved. Since industrial, mainly refinery, wastes are not normally suited to anaerobic treatment, only aerobic, or oxidation, ponds are discussed herein.

Oxidation ponds are normally about 0.8-1.2 m deep. Shallow ponds tend to have excessive weed growth, and deep ponds do not get adequate oxygen transfer to maintain aerobic conditions. Oxygen supplied by natural surface aeration and by the photosynthetic action of the algae present in the pond is used by bacteria for aerobic degradation of organic matter. In turn, algae utilize the nutrients given off by the bacteria.

Organic loadings of oxidation ponds reported in the API Design Manual range from 25-125 kg BOD/ha/d. Recommended loadings for ponds handling raw wastes are about 50 kg BOD/ha/d. For ponds downstream of other treatment processes, loadings as low as 12.5 kg BOD/ha/d should be used. Retention times range from 1 to 90 d. However, the API (1986) points out that for a pond to be effective, it should have a retention time of at least 7 d. Approximately 30 d is recommended for an organic loading of 60 kg BOD/ha/d.

The US EPA (1982) reports the following removal efficiencies for oxidation ponds: 40-95 per cent for BOD, 30-65 per cent for COD, 20-70 per cent for suspended solids, and 50-90 per cent for oil. The differences in removal efficiencies are due to the wide variations in loadings and retention times reported above.

The effectiveness of oxidation ponds is also influenced by other factors such as the temperature and turbidity of the water, as well as by the emulsions present. Any turbidity or emulsion in the water reduces the light transmission, thereby inhibiting the photosyn­thetic action of the bacteria.

Aerated Lagoons:

Aerated lagoons operate generally on the same basic principles as do oxidation ponds. Mechanical aeration equipment, associated with aerated lagoons, result in a higher concentration of bacteria than is present in oxidation ponds. Conse­quently, land requirements are less for aerated lagoons than for oxidation ponds having equal loadings. Retention time is usually 3-10 d. Removal efficiencies are 75-95 per cent for BOD, 60-85 per cent for COD, 40-65 per cent for suspended solids, 70-90 per cent for oil, 90-99 per cent for phenol, and 95-100 per cent for sulphides.

Aeration of the lagoon keeps most of the solids in suspension. One method of decreasing the suspended solids in the effluent is to have a sedimentation section in the lagoon, where the water is calm enough and the residence time is adequate to allow sedimentation of the solids. Otherwise, it may be necessary to include a settling tank.

High Rate Algal Ponds and Facultative High Rate Ponding Systems:

Municipal and organic industrial effluents can be treated with algal ponds. The methods of the treatment investigated was the high rate pond wherein the effluent is rich in algae, and the so-called integrated ponding system consisting of a facultative pond oper­ating series with a high rate pond.

The treated water can be utilized for irrigation, and the algae produced can be used for soil conditioning. Irrigation and soil conditioning can be achieved by irrigating with the treated effluent without separating the algae, or by separating the algae by sand filtration and then using the clarified effluent for irrigation and the algae-rich filter material for soil conditioning.

A pilot-plant ponding system was constructed within the premises of the Ardiya Wastewater Treatment Plant. The pilot plant, as shown in Fig. 2, consists of an oil and sand trap, 2 tanks to measure the flow into the units, 2 facultative ponds 10 x 10 x 2.5 m high, 2 oval high rate ponds 10 x 5 x 1 m deep, a cylindrical sludge digester 2 m in diameter x 4 m in height, a flow mixing and division box, and four sedimentation tanks 6 x 1 x 1.8 m deep. Each high rate pond is equipped with a paddle wheel for mixing and suspension of the algae. Plant loading could be up to 10 1/s.

Experiments were conducted with various loads at various depths and detention times varying between 8 and 40 d. The performance of the ponds were measured based on removal of the organic matter and hazardous bacteria, and algae production.

The effluent of the high rate ponds is rich in algae, which can be removed by gravitational settling in the sedimentation tanks. An alternative method is the removal of algae by slow sand filtration. For this purpose, 8 sand filters having a section of 1 x 1 m were constructed. Algae-bearing pond effluent was filtered through this filter system. The top few centimetres of the filter bed were scraped off and replaced with clear sand. The filter material which was removed was used as soil conditioner.

The results of the wastewater analysis reveal that a minimum of 50 per cent BOD removal and 25 per cent COD removal was achieved by the facultative ponds. The high BOD, COD and suspended solid values in the high rate ponds were caused by algae, which could be removed by physical treatment methods.

For this reason, treatment efficiency in the high rate ponds was expressed in terms of the BOD and COD values of the filtered effluent samples. The results obtained indicate that on the average, 95 per cent BOD and 80 per cent COD removal could be obtained through the integrated ponding system.

The results of the microbiological experiments show that significant reductions in the total coliform counts were achieved. In several experiments, the integrated system reduced the number of coliform bacteria by more than 99.9 per cent.

Sand filtration of the algae-bearing high rate pond effluents efficiently removed all the algae, reduced the BOD and COD of the final effluent, and produced an algal rich filter material which could be used as a soil conditioner and fertilizer.

An integrated ponding system is a viable alternative for wastewater treatment in arid regions. Preliminary investigations showed that vegetation in test plots increased by about 40 per cent when irrigated with pilot-plant effluent instead of ordinary tap water.

iv. Tertiary Treatment:

Tertiary treatment is usually required if the use of the treated effluent is planned for. Filtration (e.g., sand filters), chemical oxidation, and chlorination are widely used technologies. Reverse osmosis (RO) is used successfully in many coun­tries to upgrade treated effluent for special recycling purposes.

Presently this method is being investigated in Kuwait. RO and carbon adsorption can provide treated effluent for boiler feed, for example, or even a treated effluent of drinking-water quality.

Filtration:

Filtration is used only when it is necessary to obtain a very high-quality effluent or recycle stream. The effluent from activated sludge systems contains organic matter in suspended or colloidal form that can be filtered; typically, the effluent ranges in organic matter content between 5 and 50 mg/1, which can be even further reduced to around 3-20 mg/l with a granular-media filter.

The results of filtering an activated sludge effluent through a dual-media filter containing anthracite and sand have been reported. The solids in the effluent from a pilot activated sludge unit were said to have a bimodal particle size distribution. The smaller particles were in the 1-5 urn range while the larger ones were in the 50-180 µm range.

This makes the dual- or tri-media filters superior to the single-medium sand filter, where the larger particles tend to plug the filter, running times are shortened, and smaller particles are allowed to pass through. With a dual-media filter, much more efficient solids removal and longer filter runs are experienced.

Typical dual-media filters are anthracite and sand, activated carbon and sand, resin beds and sand, and resin beds and anthracite. It is recommended that a pilot study be done for any particular system because filter performance is very dependent on the particular characteristics of the specific liquid feed.

Carbon Adsorption:

There has been a tremendous amount of development in the use of activated carbon for the removal of dissolved organic material from wastewater. Carbon adsorption is normally used for the final polishing of the effluent from a biologi­cal treatment process, but it is feasible only if a very high-quality effluent is needed.

Typically, the effluent from an activated-sludge system might be treated in an adsorption unit to further reduce the BOD to 3-10 mg/1, the oil content to less than 1 mg/1, and phenol to almost zero. For most commercial-scale adsorption units, it is economical to regener­ate the spent carbon for reuse. Whenever more than 250-500 kg/d of carbon is used, a thermal regeneration system is justified.

A carbon-adsorption unit consists of the adsorbers in which the wastewater stream contacts the activated carbon bed, a transport system for moving the carbon from the adsorbers to the regenerator and back, and a regeneration system.

The adsorbers may be arranged as fixed beds in parallel, or as a moving carbon bed in which carbon is periodi­cally removed from the bottom and fresh carbon is added at the top. The API Design Manual refers to the following design parameters for adsorbers: 5-10 gpm/ft² flow rate; 10 ft minimum bed depth, and 15-38 min contact time.

Of the various methods of carbon regeneration that have been used, thermal regen­eration is the most widely applicable, because it may have multiple-hearth furnaces, rotary kilns, or fluidized-bed furnaces. Other types of carbon regeneration include chemi­cal, solvent and biological regeneration.

Chemical Oxidation:

The primary application of chemical oxidation is for the reduction of phenols and cyanides in waste streams, by oxidants such as chlorine or ozone. These processes, however, are applicable only to small, concentrated streams, for which conventional biological oxidation processes are not feasible.