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Newsletter and Technical Publications Case Study 9: Wastewater Treatment Using Overland Flow Technology Organic pollution resulting from the disposal of partially-treated sewage can cause serious river contamination. The overland treatment technology can reduce the impacts of this problem. Iraq has constructed a pilot system for treating sanitary drainage water using an advanced technology (overland flow) that relies on the infiltration capability of the soil and the water requirements of plants to treat the water. The system is operated using sewage water produced by the primary and secondary treatment units at the Al-Hilla city sewage plant, which was chosen to apply this technology because it represents the prevailing climatic conditions of most central Iraq cities (high temperature, lack of rainfall). The treatment plant that serves Al-Hilla city uses the conventional treatment technique, which includes (1) pretreatment units, (2) preliminary treatment units, (3) secondary treatment, (4) disinfection ponds, and (5) sludge treatment unit. The plant serves a population of 60,000 people, with a maximum flow reaching 12,000 m3/day. Because of the United Nations sanctions placed on Iraq, the plant does not operate at full capacity because of inadequate maintenance and lack of spare parts. As a result, the treated water still contains many untreated pollutants, which translates into an increased pollutant load to the river. This represents a serious problem in that some villages located next to the river depend directly on it as their primary water source. Thus, attention has focused on introducing a technique to prevent the large organic loads from entering the river. A new system, using the soil as a trickling filter, was proposed that includes construction of a facility between the final settling pond stage and the disinfection pond stage, for the purpose of removing detrimental microscopic organisms. The location of this proposed system within the treatment plant is illustrated in Figure 73. Technology Description The area of the treatment system is 3mx50m, with a surface slope of 2%. A distribution system is provided at the entrance of the pilot system, and a collection system at its end (Figure 74). Technology Construction Stages Constuction of this technology has gone through the following stages:
The site preparation comprises (1) site cleanup, (2) stripping and leveling of the land, and (3) planning to ensure the land is contained within the required area.
Figure 73. Location of the pilot-scale system in the Hilla treatment plant. detail
A trench of 10 cm depth and 36 cm width is excavated surrounding the pilot system. Broken bricks are then placed in the trench and covered with a thin layer of cement. A brick wall is constructed to a height of 1.2 m for a distance of 1 m, and the height gradually decreases to 0.2 m at the end of the pilot system (Figure 75).
Use of this technology requires the presence of fine texture and low permeability soil. Thus, if the selected site has unsuitable soil, the surface soil layer is removed and a fine texture soil added to a certain depth based on the type of vegetation and root depths. Because of the unsuitable soil in the selected area of Al-Hilla, silty clay with particles sizes between 0.002-0.005 mm was added. The total volume of added soil was computed on the basis of the pilot system area, as follows: (3 x 1 x 50)/2 + 3 x 1 x 1 = 78 m3 The total volume of the soil added to the area was 78 m3. The soil was carefully placed, maintaining a constant slope of 2%. The required slope is usually between 2-8%. The milder slope may create anaerobic conditions and causes the saturation of the soil, whereas the large slope results in the aeration of the soil and the reduction of the treatment time of the wastewater while flowing over the soil surface.
Figure 74. The tested overland flow system
Figure 75. Retaining wall for the pilot treatment plant Two trenches with dimensions of 0.2x0.2 m were excavated. The first trench was at the entrance of the pilot system and the second was at the end, for the purpose of water distribution and collection. An L-shaped trench with dimensions of 0.2x0.3 m was excavated to connect the collection trench to the manhole. The manhole is connected to a drainage system, as shown in Figure 76.
Figure 76. Distribution system for overland flow system The manhole is the main wastewater source, supplying the system through a pipe connected to the treatment plant’s secondary settling pond, and placed inside a manhole (size of 2x2 m and depth of 2.5 m). There are two networks of plastic pipes (5 cm diameter), the first to distribute the entering water and the second for the water leaving the system. The network consists of a main tube connected to a manhole and other side pipes, valves to control the inflow and outflow water volumes, and a sump pump with certain specifications placed inside the main manhole. Figure 76 illustrates the distribution of these pipes. An electric board also is placed near the manhole to start and stop the operation of the system. A transformer also is placed there, to protect the pump from electric current fluctuations.
The system was supplied with a drainage system at its end, consisting of a trench with a cross section area of 20x30 cm. The trench contains a plastic pipe (diameter of 7.5 cm and length of 3.5 m) surrounded with graded groups of stones. The trench is connected to a second trench of the same area to collect the water flowing from the pipe of the first trench. The second trench conveys the flowing water to the main manhole, which is connected with sub-surface pipes. These sub-surface pipes convey the water to the disinfection pond (Figure 77).
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