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This study was undertaken to investigate denitrification and nitrous oxide production in sediments and the key environmental factors influencing these within selected river systems of the LOIS (Land-Ocean Interaction Study) area in North-East England and southern Scotland. Seasonal and spatial trends were evident in both environmental and denitrification data measured monthly for 1.5 years along the Swale-Ouse system from source to tidal limits. Denitrification, measured in sediment cores using acetylene inhibition and expressed by unit area of sediment, increased with distance from source down to freshwater tidal limits. Results from a supplementary survey of the freshwater tidal reaches of the Yorkshire Ouse showed a decrease from the tidal limits. Denitrification activity showed a spring (March to May) peak, particularly in the lowland sites. The highest rate (883±134 µmol N m(^-2) h(^-1)) was measured on the River Wiske, a highly eutrophic lowland tributary to the Swale. A high degree of colinearity was evident between environmental variables, although a significant relationship between denitrification, nitrate and temperature was found through multiple regression. For comparison, measurements were made in the less populated Tweed river system. The seasonal and spatial trends evident in both the environmental and denitrification data from the River Tweed, under a more limited sampling programme, were generally consistent with those observed in the Swale-Ouse system. An intensive field investigation of 50 river sites showed that both potential denitrification rate and N(_2)O production in sediment slurries were positively correlated with nitrate water concentration, sediment water content and percentage of fine (<100 µm) sediment particles. An experimental study investigating the kinetic parameters for denitrification, found that sediment cores taken along the Swale-Ouse exhibited a saturation type curve with added nitrate. Apparent affinity and estimates of apparent maximum velocity for mixed populations of denitrifying bacteria showed an increase on moving downstream and were highest on the Wiske.

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The aims of the research project were to observe the operation of stabilization ponds with a view to investigating the value of various existing approaches used in the design of both facultative and maturation wastewater stabilization ponds and, if possible, to produce a realistic and reliable method for pond design. This latter aim was to be carried out either through the modification of existing equations or by the development of a new approach. In addition it was also intended to investigate the phenomena of dissolved oxygen and thermal stratification within stabilization ponds. The required data for this work was obtained as a result of detailed observation of the four-pond wastewater treatment installation in the Cayman Islands, British West Indies. Details of flows, flow strengths, removal of BOD and faecal coliform organisms were collected and numerous profiles of the pond contents were carried out to investigate dissolved oxygen and thermal stratification. Information concerning temperatures, solar radiation, hours of sunshine, relative humidity, rainfall, evaporation and wind speed and direction were also recorded over the three year investigational period. The most common design methods, the first-order complete-mix approach of Marais and Shaw and the modified empirical approach of McGarry and Pescod as modified by Mara and Arthur, were studied in detail using the results collected. These two design approaches were those used for the initial design of the Cayman ponds. Suggestions were then made for modifications to the first-order, complete-mix equation with regard to the values of the reaction coefficients employed for the removal of organic matter and of faecal coliform bacteria. Modifications of the equations to obtain K (the first-order reaction coefficient for either BOD or faecal coliform removal) were suggested. Following that a new empirical design approach was developed using the data collected from the operational ponds together with the climatological information. The models were developed in three steps. Firstly, matrices of correlation coefficients were produced to discover the existance of any strong correlations between any of the dependent and independent variables considered. Secondly, the Stepwise Backward Selection procedure for multiple regression (SBS) was applied to select the most relevant preliminary variables for multiple regression models. Then finially, the relevant variables were grouped in ordinary least-squares multiple regression models using a Hierarquical Multiple Regression procedure (HMR) to construct the models. As a result of this work groups of semi-empirical and empirical models were produced_ Two semi-empirical design models were developed to express K as a function of loading and climatic factors. These two equations were for the removal of BOO and faecal coliforms in facultative ponds. Two other groups of empirical design equations with a total of fourteen possible design models relating pond efficiency to factors such as loading, retention time and climatic conditions were also produced for the design of facultative and maturation ponds in the Caribbean region treating weak and saline wastewater. Also carried out were extensive field investigations into the temporal and spatial distributions of dissolved oxygen and water temperature in the ponds and as to how these varied with climatic conditions.

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