Influence of salinity on the performance of a sludge blanket clarifier

Abstract

Coagulation is an important techniquc in water treatment to remove colloidal and non-settlable fine particles successfully. These particles are forced to settle as floc with the help of chemicals and particle transport mechanism. Common coagulant dose is alum [AI2(S04h.14 H20], readily dissolves in water, is used in the pH range of 5.5 to 7.8. One of the limitations of coagulation is influence of salinity. Addition of salts interferes in the coagulation and, alum floc breaks in presence of salinity in water. CWASA faced the salinity problem in 1991 for cyclone tide and in 1995 for backwater effect of sea. To review CWASA's problem during saline intrusion and improve the situation, a physical model of sludge blanket clarifier which angle is same to Mohara Treatment Plant's clarifier has been set-up for this study. Both the clarifiers work in the same method, hence, clarifier used in the laboratory can be considered similar to Mohara's clarifier. It was found that saline intrusion problem could be minimised by controlling flow. Performance Study for different units of the plant for selected parameters, i.e., pH, turbidity, colour, Total Dissolved Solid (TDS), Total Solid (TS), alkalinity and chloride have been done for each unit of the plant and for the whole plant taking samples at inlet and outlet of each unit. The study reveals that sedimentation unit of the plant is good in removing turbidity and Total Suspended Solid (TSS). Coagulation unit's (Flash mixer and Clarifier unit) performance in removing colour and turbidity is about 75-95 %. Mohara Treatment Plant is performing well in removing turbidity, colour and TS. Alkalinity, pH and chloride of finished water were always in the range comparing World Health Organisation (WHO) guideline. This study has been done at low flow (winter season) condition of river Halda. Further study of the plant is required at high flow condition and during saline intrusion. The study shows that performance of clarifier depends on discharge, angle of clarifier with horizontal and depth of sludge. Critical discharge for the clarifier used in the study when sludge blanket disintegrates initially due to concentrated force was about 936 cm3/s/m3 of sludge. Clarifier's performance decreases when salt is added at any flow. Increased salt increases density of water, hence concentrated force increases at the inlet of clarifier. Increased concentrated force increases initial dispersion of floc in the clarifier. But 134-535 cm3/s/m3 of sludge is the optimum discharge range for the clarifier used when performance is good at optimum salt value. Limiting chloride was found 1000 mg/l from limiting concentrated force of 24764 c.g.s. unit. unit at 17.5 cm sludge depth for the clarifier used in the laboratory. Maximum tolerable discharge for the clarifier used during salinity was about 893 cm3/s/m3 of sludge at minimum 500 mgll of chloride, while maximum tolerable chloride was about 3400 mg/l at minimum discharge of 80 cm3/s/m3 of sludge. Again, maximum tolerable discharge for clarifier of Mohara during salinity was about 2219 cm3/s/m3 of sludge at minimum 500 mg/l of chloride, while maximum tolerable chloride was about 6800 mg/l at minimum discharge of 80 cm3/s/m3 of sludge. Upto 3400 mgll of chloride and at minimum discharge of 80 cm3/s/m3 of sludge, clarifier's performance can be considered similar to slow mixing coagulation. CWASA could control both discharge and chloride at the flash mixer during salinity intrusion.