Development of criteria for forming and maintaining a sludge blanket in an upflow sludge blanket clarifier/pulsator

Abstract

Coagulation and flocculation is attained within a sludge (floc) blanket of an upward flow clarifier unit. In this study cohesivity of the floc blanket, measured by the indicator sludge cohesion coefficient (SCC) is used to explain the blanket characteristics and response of the blanket to variations of raw water turbidity (RWT), coagulant dose and ambient conditions. The study found that SCC is an appropriate parameter to monitor floc blanket characteristics. A satisfactory floc blanket is established when SCC varies within 0.3 – 1.3 mm/sec and the sludge volume fraction of the blanket is between 0.2 and 0.25. At RWT occurrences > 450 NTU, the blanket cohesivity reduces. Increased coagulant dose leads to restabilization of particles by charge reversal leading to reduction of blanket cohesivity. It is recommended to introduce preliminary sedimentation (prior to clarifier) to effectively treat high turbidity raw water. Beyond RWT 300 NTU optimum coagulant dose reported from SCC test is lower than that of Jar test. This will give savings in coagulants in the range of 6 - 25%. When RWT is > 300 NTU, the linear relationship established using the two parameters during the study can be used to find the optimum dose after carrying Jar test. The study found that high inflow temperature reduces blanket cohesivity and particle settling efficiency. There is a significant linear relationship between the influent temperature and the effluent quality. The particle structuring within the blanket is due to hydrodynamic forces between the particles counterbalanced by the cohesive forces. A steady floc blanket is formed when the individual particles are agglomerated and clusters are formed. Cluster formation/destruction is due to the cohesive/inertial forces between particles and/or particle clusters. With low Re (< 1) cohesive forces govern. Interstitial spaces between particles vary due to cluster formation/destruction, leading to the increase/decrease of blanket settling velocity.