In this article we will discuss about the improvement of wastewater quality of soil aquifer treatment in Kuwait.
Water Resources Division
Kuwait Institute for Scientific Research
P.O. Box: 24885, Safat 13109, Kuwait
Wastewater is an important resource in arid regions. One of the major requirements of wastewater reuse is the treatment of wastewater to a level suitable for the intended use at an acceptable cost to the community.
Conventional treatment of wastewater in wastewater treatment plants involves treatment of wastewater to either secondary or tertiary levels. Conventional wastewater treatment plants are expensive to build and operate.
Soil aquifer treatment (SAT) is a concept which represents a modern approach to an old method of wastewater reuse. The method is very similar to the rapid infiltration system, where effluent is applied to the soil at high rates (about 10- 200 cm/wk) by spreading it in basins or sprinkling. Treatment occurs as the wastewater passes through the soil matrix.
The average cost of treatment using SAT is about KD 5-10/ 1000 m3. Wastewater reuse will considerably reduce the demand for brackish groundwater or desalinated freshwater in Kuwait. Aquifer conditions are ideally suited for the implementation of SAT in Kuwait. SAT could be implemented at locations close to the existing wastewater treatment plants. The most suitable location might be at the experimental farm at Sulaibiyah.
In arid areas, wastewater is an important resource that can be used for irrigation, recreational lakes, and municipal purposes after adequate treatment. One of the major requirements of wastewater reuse is that it be treated to a level suitable for the intended use at an acceptable cost to the community.
Conventional wastewater treatment plants usually have three stages of treatment, namely, primary, secondary and tertiary levels of treatment. In primary treatment, physical operations such as screening and sedimentation are used to remove the floating and settle-able solids present in the wastewater.
In secondary treatment, biological and chemical processes are used to remove most of the organic matter. In tertiary treatment, an additional combination of unit operations and processes are used to remove constituents such as nitrogen and phosphorous. The cost of conventional treatment of wastewater varies considerably and usually ranges from KD 40-200/1000 m3, depending on the size of the plant.
Three major problems impede the reuse of treated wastewater produced by conventional wastewater treatment plants.
(1) The high cost of treatment,
(2) The lack of buffer storage, and
(3) The psychological disadvantage of the term wastewater.
It is expensive to produce high quality, treated wastewater that can be used for irrigation or landscaping without restriction. Production of wastewater is normally constant, irrespective of seasons. The demand for treated wastewater varies with the seasons, and hence, a buffer storage is required for the storage of treated wastewater produced when the supply exceeds demand.
The quality of wastewater treated in conventional plants can be very high, but it is still called wastewater and hence, is disliked by many for psychological reasons.
Land treatment systems for the treatment of wastewater in use for several hundred years overcome some of the limitations of conventional treatment plants. Land treatment of wastewater involves the use of plants, soil surface and the soil matrix for waste treatment. The three principal processes of land treatment of wastewater are irrigation, rapid infiltration and overland flow.
Table 1 lists treated effluent quality from land treatment systems and tertiary treated effluent quality from the Ardiya treatment plant.
NB: all amounts in parts per million
The method of treatment of wastewater proposed here is developed from land treatment systems. It incorporates the innovative soil aquifer treatment (SAT) concept, which represents a modern approach to an old method of wastewater reuse. The method has been used in many countries and in particular in Phoenix, Arizona where the treatment process has been in use since the early 60s and the results has been widely publicized.
The present paper examines the suitability and usefulness of the SAT method for Kuwait.
Soil Aquifer Treatment (SAT):
SAT systems operate by the infiltration of wastewater effluent through the vadose zone (Fig. 1). Water quality improves from removal mechanisms in the soil that include filtration, biological degradation, physical adsorption, ion exchange, and precipitation. These mechanisms can be very effective in removing or reducing nitrogen, phosphorous, biochemical oxygen demand (BOD), suspended solids, organic compounds and trace metals.
The removal of nitrogen and organic compounds (BOD) can be a continues regenerative process that relies primarily on biodegradation. Removal of trace metals, phosphorous, and non-biodegradable organics occurs by physical and chemical removal mechanisms that have limited capacity. Efficiencies for the removal of specific water quality constituents vary widely with the type of soil, level of effluent pre-treatment, loading rate, loading cycle, and temperature.
During SAT, cyclic flooding/drying of the basins is necessary both to improve infiltration rates and to control aerobic/anoxic conditions in the soil. The basins are flooded until infiltration rates decrease due to the development of a surface clogging layer. During flooding, anoxic conditions develop in the upper strata of the soil and aerobic biological activity is limited.
Drying of the soil surface desiccates the clogging layer, which is then removed by natural processes or is physically removed. This aerates the soil allowing for aerobic biological activity and improves infiltration rates when flooding of the basins resumes.
Wastewater in Kuwait:
The total volume of wastewater produced in Kuwait during 1989 was about 250 Ml/d. This was treated in wastewater treatment plants in Ardiya, Riqqa and at Jahra. About 91 Ml/d of treated wastewater is reused for irrigation at the experimental farming station at Sulaibiyah.
The treated wastewater can be supplied to Sulaibiyah from all the three wastewater treatment plants. The rest of the wastewater treated is discharged to the sea. The quality of wastewater discharged to the sea varies between the secondary and tertiary levels.
The total consumption of brackish water in urban areas during 1989 was about 224 Ml/d. If the wastewater produced in Kuwait can be treated to a level acceptable to the public for unrestricted use, then treated wastewater can replace the usage of brackish water, and to a limited extent, freshwater for agricultural and landscaping purposes.
Soil Aquifer Treatment (SAT) in Kuwait:
Aquifer conditions in Kuwait are generally suited for the implementation of SAT.
Optimum conditions for the implementation of the SAT system are:
(1) Depth to water table levels between 3-30 m.
(2) Unsaturated sections free from major impervious layers,
(3) Close proximity to the source of the wastewater, and
(4) Saline native groundwater quality with total dissolved solids in excess of 10,000 ppm.
Most of these conditions are satisfied at locations close to the existing treatment plants. Probably the most suitable location for the implementation of SAT is the present experimental farming region at Sulaibiyah. The depth to the water table at Sulaibiyah is 20-25 m. The groundwater quality varies between brackish and saline. The existing network of pipes allows transport of treated wastewater from all the three wastewater treatment plants to Sulaibiyah.
The injection of secondary/tertiary treated wastewater could be either by infiltration basins (Fig. 2) or dry injection well (Fig. 3). Assuming loading rates similar to those at Phoenix, Arizona, a 500 x 500 m area will be required for the recharge of about 250 Ml/d of wastewater. Alternatively a total of 400 dry injection wells spread over an area of about 200 × 200 m could be used to recharge the wastewater.