Flocculants and Sediment Capping for Phosphorus Management

Abstract

Flocculation and sediment capping are two of the tools in the manager’s toolbox for lake restoration. For them to be used effectively requires an understanding of the in-lake processes that control the recycling of nutrients from the legacy stored in the sediments and the interactions with microbial communities and aquatic plants. The form of the nutrient is important, and there are natural in-lake processes that can influence the chemical intervention applied. For example, the chemical control of phosphorus (P) with P-inactivation agents in a lake is only applicable to soluble reactive phosphorus (SRP), i.e. phosphate, and not particulate or organic forms of P. Consequently, particulate forms, including detritus, fine sediment, and algal cells, need to be managed with flocculants which remove them from the water column to the sediments. A range of commercially available flocculation agents as well as passive and active sediment capping agents targeting P are discussed. Future prospects for chemical lake restoration methods are likely to include flocking and locking, i.e. using flocculation to settle the insoluble forms of P then locking that P in the sediments with an active barrier of P-inactivation agent as a sediment cap. Chemical lake restoration methods are not a panacea, but they are an important part of an integrated management plan that includes remedial action in the catchment.