WaterSubject

The Gaza Strip is a densely populated area. More than 1 200 000 inhabitants live in an area of only 365 km². The environmental situation has deteriorated owing to the difficulties and constraints associated with the ongoing political situation in the region. Three wastewater treatment plants have been constructed in the area to serve the population. Stabilization ponds and aerated lagoons are the current wastewater treatment systems. The existing plants are overloaded and poorly managed, with the result that the effluent quality is slightly better than that of the influent. Sludge does not generally receive any treatment other than being removed from ponds and then spread on the ground in open areas around the treatment plants, creating a serious environmental health risk. Recently, sand drying beds were constructed at one treatment plant for sludge dewatering. The system failed to solve the problem of sludge dewatering and stabilization, as the beds did not perform well. The construction of three new treatment plants is now being planned to serve the whole population of the Gaza Strip. Activated sludge and extended aeration are the proposed systems for the new treatment plants. It is estimated that by 2025, more than 11 000 m³ of sludge will be generated daily in the Gaza Strip, with a suspended solids (SS) concentration of approximately 1%. Sophisticated systems for sludge treatment in the area are not feasible. The ability of residents to pay is limited, and the human skills needed to deal with high technology are not available. Land filling is not recommended as land is scarce within the Gaza Strip, and no more land is available for disposal of sludge to landfill. Low-cost systems, such as reed beds, could be a solution to the sludge disposal problem.

‘It is not easy to find a solution, which allows good management of sludge at reasonable cost’. A reed bed system for sludge dewatering is an innovative process, being a combination of a traditional sludge drying bed and natural wetland. Reed beds are widely used for sludge treatment throughout Europe, Asia and Australia, and in more than 50 locations in the United States. Reed bed technology features low construction costs and minimal day-to-day operation and maintenance costs. The system reduces the water content of sludges, minimizes the quantity of solids, and provides sufficient storage time for stabilization of bio-solids before disposal.

Reeds act in many ways to alter the character of solids and metals present in the sludge. Firstly, their root system encourages oxygen to enter the sludge around the roots, which boosts the population and activity of naturally occurring microorganisms, which in turn mineralize the sludge. Secondly, the plants grow rapidly in this nutrient-rich medium and absorb some of the minerals, in addition to drawing water from the sludge. Thirdly, roots extend from the reed stems into the bio-solids, creating a system of channels in the bio-solids, encouraging continuous drainage and preventing the formation of a semi-impervious bio-solids layer, which is typical in unplanted beds. Meanwhile, the processes of evaporation, drainage and plant uptake combine to transform the sludge into a stable humus-like fertilizer material, which can be used either to seal sanitary landfill cells or as a soil conditioner. Although there has been much research into the performance of reed bed systems, there is still need for further investigation into the design and performance of reed bed systems in specific locations.

This paper presents the results of a reed bed system constructed in the Gaza Strip and monitored for 3 years.

The pilot system consists of two beds, each having a base area, in plan, of 200 m², and with concrete banks sloping at 1 : 3. One bed was planted with the reed Phragmites australis and the other was left unplanted. The beds were constructed on the same site as the Gaza City wastewater treatment plant. This location was selected because of the availability of land and the possibility of using sludge from the treatment plant. The beds were open (uncovered), and each bed was provided with a drainage system at the base, consisting of perforated unplasticized polyvinyl chloride pipes of 200 mm diameter surrounded by gravel 5–7 cm in size. The bed media consisted of three layers of aggregate. The bottom layer was 20 cm deep, with stones of 3–5 cm diameter. The second layer was 20 cm deep, with stones of 1–3 cm diameter. The third (top) layer was 10 cm deep, with stones of <1 cm diameter. On the top of the three layers, a 20 cm deep layer of sand was laid to a slope of 1 : 50. The bottom of each bed was sealed with a ‘Hypalon’ lining. Hypalon is an impermeable geotextile, which is used to prevent water from percolating through the base of the bed. It shows a section through the bed base and the edge of a bed showing the configuration of drainage and gravel layers. The influent system was a 200 mm diameter pressure pipe provided with a current meter, so that flows to the drying beds could be measured. The pressure pipe distributed sludge to the two beds through inlet pipes fitted with valves. Beds were loaded with sludge frequently by controlling the valves installed at the inlet pipe to each bed. The outflow from the drainage system led to a collection chamber where the leachate was collected by gravity and then pumped back to the treatment plant for further treatment. The outlet was provided with a water meter to measure the quantity of infiltrated water draining from the beds.