The Efficiency of P-Removal from Natural Waters with Sorbents Placed in Water Permeable Nonwovens

Abstract

An important aspect of lake restoration is the reduction of phosphorus (P) concentrations in water. To achieve this effect, various methods are used, including chemical inactivation by means of the addition of different substances into the water column or sediment. Although the efficiency of chemical methods is high, there are numerous concerns related to the introduction of large amounts of atypical compounds to aquatic ecosystems, eg. the possibility of the appearance of their toxic forms, such as aluminium as a result of pH changes. The aim of the research was to evaluate the possibility of using water-permeable nonwovens as P sorbent carriers. A bag made of nonwoven is filled with the sorbing material. When placed in the water column, sorbent present in the bag reduces orthophosphate (PO43-) concentration in the water due to the water inflow into the bag. The bag is then removed from the water together with the sorbent and sorbed PO43-, which enables permanent removal of P and sorbent from the water reservoirs. Three synthetic sorbents were used in the study: SINOBENT (form: paste, composition: calcium, iron and magnesium embedded in a bentonite carrier), HYDROPHOSBIND (hydrogel capsules, biopolymer with embedded FeCl3), precipitated calcite (PCC, powder, CaCO3) and one natural sorbent: ground calcite (GCC, powder, CaCO3). Water samples for the study were taken from the eutrophic lake from the epi- and hypolimnion zone. Sorbents efficiency was tested in low (0.043 and 0.067 mg PO43-/L) and high (0.230 and 0.540 mg PO43-/L) concentrations at two temperatures (6°C and 20°C). Sorption of PO43- was tested for the sorbents placed in a bag made of permeable nonwovens. The dose of materials was 1g/L, except for HYDROPHOSBIND (5 g/L). Additional experiments were carried out with the addition of effective microorganisms to check their effect of the sorption behaviour of the tested sorbents. An unambiguous PO43- loss from the solution was observed for three materials: PCC, GCC and HYDROPHOSBIND. Given for 1g of the sorbent, the highest losses (19-79%) were found in PCC and the lowest (5-13%) in HYDROPHOSBIND treatments. For these materials, the effectiveness of PO43- removal raised with rising temperature and the initial PO43- concentration. The addition of probiotic organisms favoured the reduction of PO43- concentration, especially in the presence of PCC. The addition of SINOBENT usually resulted in an increase in PO43- concentration and pH decrease even to pH=6.45.