A Laboratory-Scale Investigation into the Use of Powdered Iron for In situ Removal of Phosphorus from Treated Sewage Effluent with Constructed Wetlands

Incorporation of powdered iron into the root bed of experimental constructed wetlands removes phosphorus from treated sewage to levels that easily meet government guidelines for discharge into natural water bodies. Various combinations of powdered iron and sand were used to determine the best arrangement that removes the largest amount of phosphorus. Macrophytes fulfill a beneficial function by delivering oxygen to the root zone and by improving drainage in the wetland. In these wetlands, cattails (Typha latifolia L.) also take up phosphorus during growth and store significant quantities of it in their leaves. Phosphorus is a primary nutrient for aquatic plant growth and is a major cause of eutrophication in rivers and lakes. Small communities are often faced with the expense of major public works to bring phosphorus discharges into compliance with local regulations. In terms of phosphorus removal, a combination of primary and secondary sewage treatment systems can only remove up to 20 to 30% of the input loading (Hocking 1985). When the effluent from the secondary process is not acceptable, tertiary or advanced treatments must be used. Two commonly used methods utilized are shallow lagoons or chemical coagulation. Shallow lagoons have good sunlight penetration and oxygen exchange to the bed, which allows for continued microbial action as well as photosynthetic waste utilization. It was the photosynthetic removal of phosphates that led to the concept of using shallow lagoons for nutrient removal. In order to maintain phosphorus removal, a system to remove algae must be employed. Shallow lagoons have the advantage of being simple in concept, but the processes require relatively long retention times to be effective and necessitate significant land areas for their use. In addition, complications can arise from imbalances in nutrient ratios, proper pH, and temperature. The predominant reactions involving iron and aluminum ions with water are hydrolysis reactions to form metal hydroxides. The removal of phosphorus is considered to occur as a co-precipitation, or adsorption onto the hydroxides that form. These coagulants are added at various