Modelling phosphorus removal capacity and longevity in land treatment systems.

The wastewater industry is facing an increase in regulatory pressure to reduce excessive phosphorus (P) release to rivers. To address this issue there is a need for sustainable treatments, especially in small sewage treatment works (STW) (< 2000 inhabitants) which constitute 75 % of the systems in the UK. Land treatment systems (LTS) have long been used to treat wastewater, principally as a tertiary treatment at small rural sites. LTS are able to remove phosphorus but there are some uncertainties regarding their treatment capability. This research aims to address these uncertainties by developing improved estimations of P removal potential and longevity in LTS in the context of a sustainable water industry. First, the existing knowledge and tools available in long-term P removal behaviour are reviewed. Second, the soil P removal efficiency and accumulation in an LTS case study are estimated. Finally, the case study is used to develop a conceptual and mathematical model of P dynamics in LTS integrating water and solute transport in soils. The model is used to explain system behaviour in order to estimate system longevity under different soil conditions and management scenarios. This research demonstrates that LTS can contribute to P removal for a number of years as a tertiary treatment for small STW and that through a combination of hydrological and solute transport modelling it is possible to get a prediction of P removal and longevity of LTS. Results indicate the need for an alternative methodology to assess P removal capacity that does not only rely on inputs and outputs while able to detect if the system is saturated and the need of improved methodological assessment of sorption processes over time. Further development of the current model to reduce limitations, mainly related to vegetation management and P transformation in the different soil pools, will support LTS contribution to the sustainable management of wastewater treatments and the enhancement of the ecosystem services that they provide.