The effect of applied direct current on biological phosphorus uptake

Abstract

An increase in metabolic activity of bacteria capable of accumulating excess phosphorus may also improve the phosphorus removal efficiency of the biological phosphorus removal system. It has been observed that the amount of intracellular biochemical energy (ATP) in bacterial cells has increased when they were exposed to pH or electrical stress. The utilization of the surplus intracellular energy generated by the stress may be utilized to stimulate the metabolic activity of bacterial cells. It was hypothesized that the introduction of direct current into mixed liquor might also increase the metabolic activity of bacteria capable of accumulating excess phosphorus. A direct current with 6 V and 200 mA was applied intermittently for one minute in every five. The direct current was introduced at the anaerobic step of a Phoredox type laboratory scale continuous flow system. Graphite electrodes were used to introduce the current. To separate the effect of the electric current from that caused by other environmental factors a control system was run in parallel using the same operational parameters. Synthetic wastewater with the same composition was used as feed for both systems. As a result of the intermittent application of direct current, the rate of phosphorus accumulation in activated sludge increased by 30% during the start-up period of the biological system and by 11 % during steady state conditions. The applied direct current changed the movement and concentration of potassium ions. The pH of the mixed liquor in the anaerobic reactor dropped to 4 as soon as the direct current was introduced and returned to the initial pH 6 value when the current flow ceased. The number of viable cells in the activated sludge decreased by a factor of 2–5 as a result of the electrification. The reduction observed in viable cell count did not influence the overall organic carbon removal efficiency or the nitrification-denitrification processes. A hypothetical model was developed which combines known biological reactions with results observed during the research. The model summarizes the effects of direct current on biological phosphorus uptake. This work provides insight into a method of enhancing the biological removal of phosphorus from a wastewater stream without adding chemicals. It has many practical applications where electric power is available but chemical addition is unacceptable (water reuse), the cost is prohibitive or the appropriate chemicals are locally unavailable.