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AbstractDuring recent years it has become clear that, particularly to protect the quality of sea waters, nitrogen and phosphorus discharges have to be substantially reduced. Nitrogen reduction can take place by conventional biological treatment. However, the problem can be partly, or perhaps completely, resolved by treating the returned liquors resulting from sludge dewatering. These normally create a substantial load on sewagetreatment works; in fact the nitrogen in the returned liquors can contribute 15–25% of the total nitrogen load entering the works. It therefore seems appropriate, particularly with a view to future nitrogenreduction requirements, to treat the returned liquors before they are returned to the works'inlet. Since 1987, Watergroup has been working on these problems, and the company now has full‐scale plants at Frederikshavn, Denmark (population equivalent 130 000) and Eslöv, Sweden (population equivalent 250 000). Normally this treatment can be carried out at a considerably lower cost, per kg nitrogen removed, than when applying traditional methods. An additional advantage is that the method makes it possible to reuse the nitrogen content of the returned liquor, e.g. in the form of ammonium sulphate which is an excellent fertilizer.

Abstract. Sustaining water quality is an important component of coastal resilience. Floodwaters deliver reactive nitrogen (including NOx) to sensitive aquatic systems and can diminish water quality. Coastal habitats in flooded areas can be effective at removing reactive nitrogen through denitrification (DNF). However, less is known about this biogeochemical process in urbanized environments. This study assessed the nitrogen removal capabilities of flooded habitats along an urban estuarine coastline in the upper Neuse River estuary, NC, USA, under two nitrate concentrations (16.8 and 52.3 µM NOx, respectively). We also determined how storm characteristics (e.g., precipitation and wind) affect water column NOx concentrations and consequently DNF by flooded habitats. Continuous flow sediment core incubation experiments quantified gas and nutrient fluxes across the sediment–water interface in marsh, swamp forest, undeveloped open space, stormwater pond, and shallow subtidal sediments. All habitats exhibited net DNF. Additionally, all habitats increased DNF rates under elevated nitrate conditions compared to low nitrate. Structured habitats with high-sediment organic matter had higher nitrogen removal capacity than unstructured, low-sediment organic matter habitats. High-precipitation–high-wind-storm events produced NOx concentrations significantly lower than other types of storms (e.g., low-precipitation–high-wind, high-wind–low-precipitation, low-wind–low-precipitation), which likely results in relatively low DNF rates by flooded habitats and low removal percentages of total dissolved nitrogen loads. These results demonstrate the importance of natural systems to water quality in urbanized coastal areas subject to flooding.

ABSTRACTThe paper reviews the current capabilities of processes for phosphorus and nitrogen removal in relation to the EC urban waste water treatment Directive. The Directive is briefly described and the implications for the UK are discussed. This is followed by a review of biological and chemical processes for nutrient removal with special reference to their current state of application and performance. Particular attention is paid to processes for biological phosphorus removal which, in the past, was said to be less reliable than the chemical options but which is now being more widely applied in other parts of Europe as well as South Africa and North America. Processes for modifying both activated sludge and biological filter systems to meet the Directive are briefly described.

Stricter legislations on nitrogen removal together with increasing wastewater loads have successively increased the need for optimization and control of activated sludge processes. The aim of this thesis is primarily to propose and illustrate methods for modelling and control of activated sludge processes, and thus possibly improve process efficiency. The first part of this thesis describes a JAVA based simulator of the activated sludge process. Its features are discussed, and some control strategies are illustrated. The second part of the thesis presents a simple model based feedforward controller for controlling the nitrate level by using an external carbon source. Several simulation examples are used to illustrate the control law. In the third part, a model based strategy for control of the ammonium level by manipulating the aeration volume in the activated sludge process is presented. The strategy is based on exact linearization combined with some logical programming rules in order to deal with discrete volume changes. The control law is also evaluated in simulations.

AbstractThis study examines the removal of nitrogen from nitrate, which accumulates in the anammox process, during the heterotrophic denitrification (HD) reaction using organic compounds that were originally contained in the influent as electron donors. The experimental system comprises a single partial‐nitritation/anammox (PNA) reactor and HD reactor for biological oxygen demand (BOD) reduction that is placed in series before the PNA reactor. A portion of the effluent from the PNA reactor was allowed to flow back to the HD reactor. The nitrate concentration in the effluent from the PNA reactor reduced as the recirculation ratio increased and the maximum nitrogen removal efficiency (NRE) was optimized at a hydraulic retention time (HRT) of 3.0 hours in the HD reactor out of a total HRT of 36 hours at 4 backflow/influent, demonstrating an NRE of 95.4% that represented an enhancement of 1.24 times the NRE without recirculation in the flow path.

ABSTRACTIn order to protect the sensitive waters in Tolo harbour (Hong Kong), Sha Tin and Tai Po sewage‐treatment works were first designed to remove 70% of the nitrogen load from the sewage. Since then, due to continuing serious eutrophication problems in the harbour, both plants have been modified to increase the removal efficiency to 90%.The modifications were based on the Bardenpho process. However, the designers of the two plants adopted different approaches to process intensity, complexity and control of bacterial foam. At Sha Tin, the average monthly results have shown an increase of total nitrogen removal from 60–70% to about 80% since its completion. The addition of methanol was found to be ineffective on further enhancement of the denitrification rate due to difficulties in the apportioning of the second anoxic zone. The overall monthly results for Tai Po have also shown an increase in the removal rate to about 80%, even though 90% was achieved for a short period of time. The major problem encountered at the latter plant was that the process design did not provide an effective control on bacterial foaming, which had affected the smooth operation of the process.