Water Journal : Water Journal December 2011
technical features 78 DECEMBER 2011 water small wastewater systems Denitrification Nitrification in the sand filters generates nitrates. To achieve water reuse standards, the concentration of nitrates must be significantly reduced. At the Drarga and North Shouneh WWTPs, denitrification is required because these facilities provide year-round water reuse. At Drarga, denitrification is accomplished in denitrification lagoons, while at North Shouneh concrete denitrification reactors are located immediately downstream of the anaerobic/settling digestion reactors. Nitrified effluents from the sand filters for both facilities are recycled to the denitrification processes where an anoxic environment is maintained for the conversion of nitrate-nitrogen to nitrogen gas by heterotrophic organisms. Carbon required for denitrification at both facilities is provided by the influent wastewater. Effluent polishing In three of the facilities, discharge from the sand filters flows through reed beds (man-made constructed wetlands) to further reduce organics, nitrogen, solids and pathogens. Reeds from these beds can be harvested and sold several times per year. Discussion of Individual Treatment Plants Ben Sergao pilot plant, Morocco A pilot WWTP was built in the southern Agadir suburb of Ben Sergao in 1989 through a combination of local, state and French foreign aid funding. The goal of the Ben Sergao pilot project was to demonstrate the feasibility of using anaerobic lagoons and intermittent sand filters for domestic wastewater treatment in an effort to minimise operational and maintenance costs by limiting the use of complex mechanical and electrical equipment. The pilot plant has a treatment capacity of 750m3/d and features an influent manually-cleaned bar screen, a 1,500m2 anaerobic lagoon, a 250m3 flow-holding basin and five 1,500m3 intermittent sand filters. The flow-holding basin is used to control the flow discharged to the sand filters. Each sand filter is active for three days and at rest for two days. Therefore, the treatment facility is operated with a 2.0 day hydraulic retention time (HRT) in the anaerobic lagoon and an intermittent sand filter dosing rate of 167mm. A process flow schematic of Ben Sergao is shown in Figure 1. Plant performance data is presented in Table 1. The Ben Sergao WWTP is within close vicinity to the Municipality of Drarga. As a result of the successful operation of the Ben Sergao WWTP, local and regional government officials desired to use the Ben Sergao WWTP as a model for the proposed Drarga wastewater treatment facility. Operational data and experiences from the Ben Sergao facility were used to develop the design for the Drarga facility. The familiarity of local Drarga officials with the Ben Sergao facility simplified "buy-in" to the project, while the close proximity of the Drarga facility to the Ben Sergao facility provided training opportunities for Drarga operational staff and laboratory personnel. Drarga, Morocco The municipality of Drarga is located in a semi-arid region near the coast of southwest Morocco. Water resources in the area had been exhausted by construction of the Abdelmoumen Dam, continuing drought conditions, increased population and the excessive use of groundwater. As part of the Water Resources Sustainability (WRS) project, jointly funded by the United States Agency for International Development (USAID) and the Moroccan Ministry of the Environment, a wastewater treatment facility and water reuse system for the area was developed. The wastewater treatment facility was designed in compliance with the World Health Organization standards for unrestricted agricultural irrigation water. Unit processes for the wastewater treatment facility consisted of influent screening, grit removal, two 918m2 anaerobic lagoons, two 736m2 denitrification lagoons, 101,560m2 recirculating sand filters and two 2,900m2 reed beds. The anaerobic and denitrification lagoons were designed for a 3.0 day and 2.4 day HRT, respectively. Each sand filter is dosed once every five dosing periods. There are three dosing periods each day at a rate of 230mm per fill. The recycle rate from the sand filter effluent to the denitrification lagoons is 1,470m3/d. Table 1. Ben Sergao WWTP performance (1995). Parameter Influent Anaerobic Lagoon Effluent Final Effluent Removal Efficiency BOD5 (mg/L) 347 132 5.5 98.4% COD (mg/L) 1,072 450 45 95.7% TSS (mg/L) 440 132 1.9 99.6% TKN (mg/L) 174 126 17 90.2% NO3- 0 0 157.1 N/A TP (mg/L) 27.7 4.5 17.5 36.8% Fecal Coliforms (MPN/100 mL) 5.1 x 109 2.1 x 108 134 7.5 Log Units Nematodes (No./L) 139 32 0 100% Anaerobic Lagoon Influent Screen Plant Bypass Plant Influent Flow Holding Basin Intermittent Sand Filters Diversion Structure Effluent Flow Monitoring Station Plant Effluent Figure 1. Process flow schematic of the Ben Sergao wastewater treatment facility. Recirculating Sand Filters Plant Influent Screening & Grit Removal Reed Beds Denitrification Lagoons Anaerobic Lagoons Effluent Holding Basins Reuse Water for Crop Irrigation Nitrate Recycle Figure 2. Drarga Wastewater Treatment Plant Schematic. Evaporation Ponds Septage Receiving Station Intermittent Settling/Digestion Reactors Reed Beds Filter Dosing Basin Anaerobic/Settling Lagoons Figure 3. Schematic of the Shobak Jordan WWTP. Operators monitoring effluent quality at Shobak.
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