TREATMENT AND RECYCLING OF SAND FILTER BACKWASH WATER PRODUCED FROM WATER TREATMENT PLANT

Abstract

A great amount (2–10%) of wastewater is produced by sand filter backwashing in conventional drinking water treatment plants around the world. Membrane technology, especially in micro- and ultra-filtration processes, offers a suitable treatment for recycling sand filter backwash water (SFBW) to the plant. In the Buraidah water treatment plant, Saudi Arabia, approximately 1,000–10,000 ݉ଷ /d (1–10% of total production) of SFBW is generated. The produced SFBW is disposed to the evaporation pond without any treatment. The backwash water in the evaporation pond may cause the contamination of groundwater through leaching ofheavy metals from pond sediments. In addition, growing urbanization and industrialization trends as well as limited groundwater resources are likely to lead to water scarcity in the Kingdom of Saudi Arabia (KSA) in the near future. Therefore, the reuse of the backwash wastewater is certainly required to conserve these limited water resources in the KSA. The purpose of the current study is to investigate the feasibility of a low-cost ceramic filter coupled with a coagulation and flocculation process to treat the SFBW. Preliminary results of SFBW quality indicate a very high value of turbidity, iron (Fe) and manganese (Mn) concentrations that must be treated before recycling for any purpose. The average turbidity value (516 NTU) was found to be 100 times higher than the WHO standard values, whereas Fe (60 mg/L) and Mn (5.5 mg/L) concentrations were 600 and 55 times, respectively, more than WHO standard values. A low cost-ceramic filter was manufactured by readily available materials including clay soil and rice bran. The cylindrical shaped (10 cm outer diameter and 6 cm inner diameter and 10 cm height) filter was made by mixing of clay soil and rice barn at a ratio of 80: 20 (by weight) with water, followed by burring at a temperature of 9000C. iii Aluminum sulfate (Alum), was used as coagulant as for the pre-treatment of SFBW. Optimum dose of alum was determined by jar test experiments. Experimental results showed that 20 mg/L of Alum is the optimum dose for maximum removal of turbidity, Fe and Mn from SFBW. Laboratory scale continuous dead-end ceramic filtration experiments were carried out to optimize the filtration system. SFBW samples were pre-coagulated with optimum dose of alum before continuous filtration. The filtration experiments were run with various trans-membrane pressure (TMP) and monitoring the permeate flux and permeate quality. Results showed that in 19 days of continuous filtration, the flux was constant and did not significantly decline at operating pressure 0.60, 1.5 and 3.0 KPa. The corresponding fluxes were measured 480, 1000 and 2000 L/݉ଶ /d at operating pressure 0.60, 1.5 and 3.0 KPa, respectively. Turbidity, iron (Fe) and manganese (Mn) removal were achieved around 99% at those operating pressures. However, when the pressure increase to 4.5 KPa, decline of flux from 3000 L/m2/d to 1170 L/݉ଶ /d were observed due to fouling occur in the filter. The fouling mechanism at 4.5 KPa operating pressure was interpreted by analyzing the experimental data with different pore blocking and cake layer formation model. The complete pore blocking model was the most appropriate to explain the insight mechanism of flux decline; it demonstrated that the particles in the feed water neither form a cake layer nor cause any pore blocking of the filter at low TMP. However, complete pore blocking occurred and the flux was eventually declined at 4.5 Kpa pressure. From the continuous filtration data, the optimum operating pressure and permeate flux were calculated to be 3.0 kPa and 2,000 L/݉ଶ /d, respectively. The permeate water quality (except microbiology) that was obtained during filtration is acceptable to recycle according to the standard limit of SASO (Saudi Arabia Standard Organization) and WHO drinking water quality. The filter can be operated with 3–30 folds lower TMP than that of the previously studied commercial membrane. Economics studies of the low-cost ceramic filtration system of the current study revealed that annualized cost for the treatment of SFBW was varied from 0.015–0.021 US$/݉ଷ of water production. The low-cost ceramic filter coupled with the coagulation and flocculation process would be a low-cost, low-energy consumption and a highly efficient option for recycling SFBW.