Understanding P cycling in sediments within Constructed Wetlands.

Constructed wetlands (CW) are considered to be an effective and environmentally friendly way of reducing the quantity of undesirable nutrients (P and N) discharged from point sources of pollution. Such discharges cause undesirable extra productivity i.e. eutrophication, in the receiving water body. In 1995 a CW was built at Slimbridge to treat waste water from the bird reserve before it is discharged into the adjacent River Severn. It consists of a settlement lagoon, a set of reed beds and a discharge pond. Initially not only were the nutrient discharges dramatically reduced but a new habitat for wild birds and otters was created. However recent data have shown that the CW is no longer working efficiently and that the sediments of the settlement lagoon in particular is now acting as a source of P (the critical limiting nutrient) particularly during summer, the period of greatest eutrophication risk. In organic rich sediments, labile organic P (POP) is broken down by microbial respiration, releasing phosphate. Commonly, P is also recycled by being adsorbed onto iron oxyhydroxides and released when those sediments become anaerobic. Recent studies have suggested the existence of a previously unconsidered and potentially important process, namely recycled of P by anaerobic bacteria themselves. Ingalls and co-workers, using ultrahigh resolution microscopic techniques, have seen polyphosphate granules within bacterial cells in organic rich surface sediments. High levels of P (up to 19% dry weight) have been shown to accumulate in biofilms of denitrifying bacteria and the phosphate is released when these cells become anaerobic. Furthermore it has been suggested that if dissolved calcium is present, these cells can act as sites for apatite precipitation, thus eliminating biologically available P from the system. Objectives and Hypotheses in this study: The aim of this study is to understand the seasonal nature of the biogeochemical processes occurring in the sediments of settlement lagoon including important processes previously not considered. This will not only develop new fundamental understandings of P cycling in such freshwater sediments but also enable us to develop an efficient and cheap method to fix the P within the CW and prevent unacceptable pollution downstream. We hypothesise that: (1) P from the waste stream at Slimbridge accumulates as labile organic P, some iron-bound P and polyphosphate granules within heterotrophic bacteria (including denitrifying bacteria) in the settlement pond in winter. (2) As the temperature warms up, the rate of bacterial respiration increases and phosphate is released into the pore waters by heterotrophic respiration. As the redox boundary migrates towards the sediment surface, P is released from ironbound-P and from polyphosphate granules. (3) It will be possible to fix this phosphate as non-bioavailable apatite by altering the chemical environment around the bacterial cells by increasing the calcium concentration. This alteration will be more effective (and expensive) than alternative systems, e.g. the limestone weir currently used, because it will utilize our knowledge of the chemistry in the immediate vicinity of the bacterial cells. Experimental design: We will carry out nutrient budgets of the lagoon together with detailed study of the sediment and its pore waters. We will use conventional and high resolution gel sampling & analysis and ultra-high resolution electron microscopy to understand seasonal changes in biogeochemical processes in the sediment of the settlement lagoon. A series of microcosm experiments will be carried out; a) to understand the exact nature of the seasonal release of P from the sediment b) to develop the optimum way of fixing P as apatite by adding calcium to the sediments in different forms and conditions c) to test this new treatment to fix P as non-bioavailable apatite within the sediment. by carrying out in-situ pilot experiments