Enhancement of sludge settling with chemical additives.

Solids in aeration tank effluents contribute to the total organic content and suspended solids concentration, and violates disposal and reuse standards. Therefore, improving the settling properties of sludge helps produce an effluent with a quality that conforms to effluent standards, assuming all other conditions for good settling in the sedimentation tank are favorable. In this paper, seven chemicals that are all cationic polyelectrolytes were tested individually as additives to improve the settling properties of aeration tank effluent from a full-scale wastewater treatment plant. Pilot settling columns designed and constructed at a local wastewater treatment plant were used in this project. Batch settling tests were performed on fresh activated sludge samples that were collected before the secondary sedimentation tank and mixed with different concentrations of the additives without cumulative additions. Each additive was accessed based on its particular sludge settling properties, that is, the shape of the settling curves, the values of the hindered zone settling rate or zone settling velocity, and the stirred sludge volume index. Each additive result was compared with the corresponding properties of a plain sludge sample collected on the same day and used as a control. Substantial and dissimilar improvements were obtained and are presented in the form of graphs and values using 92 settling runs. Shorter lag-times, quicker descending speed of sludge-water interface, higher descending distance, and lower stirred sludge volume indices values were attained with the use of these additives.

Chlorine decay in drinking-water transmission and distribution systems: pipe service age effect.

Water quality can deteriorate in the transmission and distribution system beyond the treatment plant. Minimizing the potential for biological regrowth can be attained by chlorinating the finished water. While flowing through pipes, the chlorine concentration decreases for different reasons. Reaction with the pipe material itself and the reaction with both the biofilm and tubercles formed on the pipe wall are known as pipe wall demand, which may vary with pipe parameters. The aim of this paper was to assess the impact of the service age of pipes on the effective chlorine wall decay constant. Three hundred and two pipe sections of different sizes and eight different pipe materials were collected and tested for their chlorine first-order wall decay constants. The results showed that pipe service age was an important factor that must not be ignored in some pipes such as cast iron, steel, cement-lined ductile iron (CLDI), and cement-lined cast iron (CLCI) pipes especially when the bulk decay is not significant relative to the wall decay. For the range of the 55 years of pipe service age used in this study, effective wall decay constants ranged from a decrease by -92% to an increase by +431% from the corresponding values in the recently installed pipes. The effect of service age on the effective wall decay constants was most evident in cast iron pipes, whereas steel pipes were less affected. Effective chlorine wall decay for CLCI and CLDI pipes was less affected by service age as compared to steel and cast iron pipes. Chlorine wall decay constants for PVC, uPVC, and polyethylene pipes were affected negatively by pipe service age and such effect was relatively small.