Polluted Drainage Water: Natural In-Stream Remediation
The main objective of wastewater treatment is to allow human and industrial effluents to be disposed of without danger to human health or unacceptable damage to the natural environment. The quality of treated effluent used in agriculture has a great influence on the operation and performance of the wastewater/soil/plant—or, aquaculture—system. Adopting as low a level of treatment as possible is desirable in developing countries, due mainly to economic considerations and ease of operation and maintenance.
In rural areas, domestic wastewater is typically discharged directly or indirectly to drains, causing degradation of drainage water quality against the reuse of drainage water. Low-cost treatment alternatives are more obviously practical solutions for poor and remote locations. Natural wastewater treatment requires relatively low capital investment when flat land is available at a reasonable price.
Among natural treatment systems, in-stream wetlands have high potential for application in rural areas of Egypt where the treatment process takes place within the drainage area: it needs much less land, is easily maintained, can adsorb shock loads of water, requires relatively less capital, and involves lower operating costs. The use of in-stream wetlands is being used more often to mitigate the impacts of both point and non-point source pollution. Wetlands promote physical, chemical, and biological processes that attenuate and convert nutrients, which can lead to improved water quality. In-stream wetlands are created by modifying existing streams or water courses, most often through excavation. Flow paths are maximised by forcing water around islands or land wedges, and through thick vegetation or similar obstacles. Wetland outlets can be designed to limit the exit flow, thus retaining waters for longer periods. Decreased velocities can help promote the deposition of sediments and solids in the wetland, while lowering potential erosion of banks downstream.
A demonstration of such technology was initially built in the village of Al Bahow, Dakahliya, in the East Nile Delta, where 300 m3 per day of wastewater were treated through the drain path, thereby saving about 5,000 m3 per day of clean drainage water for irrigation proposes. The Al Bahow in-stream wetland is built inside the drainage area. It consists of a sedimentation pond followed by a series of floating and emergent aquatic plant cells. It has an open water reach with a control weir to adopt the treatment process at the drain outlet. The sedimentation pond removes most sediments and some of the organic loads. The plant cells remove the rest of the organic load and heavy metals, along with some of the bacteria and microbes. As a final treatment stage, the open water area utilises sunlight and air to disinfect the water and boost oxygen content.
- Water samples were collected on a bi-weekly basis from the drain inlet at the point source, just before the in-stream treatment, after the in-stream treatment and at the drain outlet. As for total suspended solids (TSS), their main removal was at the sedimentation pond, where TSS dropped from 915 to 114 mg/l, while the vegetated in-stream wetland cells reduced TSS to 70 mg/l, and to 20 mg/l at drain outlet. The sedimentation pond did not remove biochemical oxygen demand (BOD) at the same rate of TSS: BOD fell from 550 to 32 mg/l before entering the wetland cells, and further to 12 mg/l after the in-stream cells and 7 mg/l at the drain outlet.
- Some main keys to the success of this approach, compared to conventional wastewater treatment technologies are the low costs of construction, operation and maintenance, and its ‘zero’ ecological footprint. Moreover, in-stream treatment does not require skilled manpower for management, while conventional technologies require well-trained staff. The fact that it consumes zero power is another great advantage.
- Pollutant concentrations along the drain (prior to, during and after treatment), and treatment efficiency
Based on the planned retention period and aquatic species used, the performance of this in-stream wetland treatment system under Egyptian conditions is expected to be equivalent to those of primary-to-secondary conventional treatment methods.
In general, certain conditions are necessary for applying in-stream wastewater treatment. Selection procedures may be summarised as follows:
Existence of an in-stream water course
- A small-scale drain with a minimal degree of slope within the drainage network is preferable due to its small cross section and shallow water depth (less than 1 m), small discharge (small agricultural served area) and sufficient length for treatment.
Fitness of civil works
- Bottom-deepening, side-widening or building weirs or trash screens can be made easily to cope with planned treatment requirements.
Small population or remote area
- A small village or jurisdiction that is remote from any wastewater treatment facility is preferable. Moreover, the selected area should not be included in national sanitation plans covering the near future (i.e. 10-year plans). Selection might include communities that are equipped with a primary sewage treatment facility.
Sewerage collection network
- An existing pipe network to collect the area’s sewerage wastewater at the drain entrance (or along the first quarter of its length) as a point source is a necessity. Scattered sewage outlets along the drain area are prohibited, as they will compromise the effectiveness of the in-stream treatment function.
Free-flow subsurface drainage system
- Because the main function of agricultural drains is to collect subsurface drainage water via impeded pipe networks, outfalls to the drain must be free-flowing to prevent waterlogging at crop-root zones.
Existence of native aquatic plants
- In-stream treatment depends mainly on natural aquaculture species for uptake of pollutants, filtration and carrying bacterial biofilm. From an environmental point of view, it is prohibited to introduce any foreign species of flora or fauna.
Community acceptance and awareness
- Both sewerage producers and drain stakeholders should be made aware of the pollution problem, accept the proposed in-stream drain treatment, and agree terms of future self-operation and maintenance.
- EUR 120,000
Prof. Dr. Ahmed Rashed, Drainage Research Institute, NWRC