Trash Screens on Culverts in Urban Areas

T his PAPER DEALS with the performance of trash screens on culverts on urban watercourses. An assessment of the scope of problems associated with screens has been obtained from a survey of land drainage authorities in urban areas in England and Wales. The findings have been analysed and this has resulted in the development of four basic design criteria for screens. A detailed survey of the performance of some 17 sites in the river Ravensbourne catchment of the Thames Water Authority was undertaken over a number of years allowing relationships to be identified between debris amounts and types of upstream catchment area, screen/culvert area ratios, and priority of location.
The paper describes considerations to be taken when providing a screen and advocates an individual design for each location rather than installing a standard arrangement. The lack of practical information in the UK, and the high level of problems experienced overall underlines the need for proper consideration to be given to this subject. Current research work is being undertaken to obtain preferred efficient screen configurations, but it is concluded that this should complement rather than replace the need for good trash screen design.     

Risk Assessment for a Proposed Groundwater Abstraction Scheme in Strathmore, North-East Scotland: a Modelling Approach

The Lower Old Red Sandstones in northern Strathmore have favourable hydrogeological conditions for ground-water development. A scheme was proposed to abstract water in the north Esk catchment in order to augment water supplies to local villages, and a production borehole was drilled into the Edzell Sandstones to give a design yield of 2420 m³/d. However, this scheme has been abandoned because of concern that the abstraction would significantly reduce the flow in a nearby stream which is an important salmonid fishery.
This study demonstrates the potential value of using a stream-aquifer numerical model in an environmental risk assessment for the proposed scheme.     

Preliminary Results of the Anaerobic Biotreatability of the Effluent from the Wet-Air Oxidation of Sewage Sludge

Wet air oxidation and other thermal processes can be used on all sludges, whether they are raw, mixed or digested. The process sterilizes the sludge and gives a considerable reduction in volume. However, apart from the high capital costs which are associated with this technique, one of its principal disadvantages is the high strength of the (effluent) liquor.
This study investigated the feasibility of treating the liquor using an anaerobic digester, which resulted in a good reduction in BOD and a reasonable reduction in COD. The liquor was still strong, but could be returned to the inlet works of a sewage-treatment plant without causing significant organic overloading.     

Sewage Sludge Production, Treatment and Disposal in the European Union

Sludge production and disposal are entering a period of dramatic change, driven mainly by EC legislation. The urban waste water treatment Directive will result in at least 50% more sludge being produced by the end of 2005 and, during the next decade, sludge disposal to all the established outlets could become increasingly difficult or, in the case of sea disposal, will become illegal. The challenge facing the members of the European Union is how to (a) maintain cost-effective and secure methods of sludge disposal and (b) engender public confidence in all disposal and recycling options. What is required is not so much innovative technology, although this may be essential, but more innovative attitudes and approaches to promotion and defending existing outlets.
This paper describes the likely pattern of sludge production, treatment and disposal which will emerge during the implementation of recent EC directives and other national policies in Europe, and focuses on the areas and issues which require innovative approaches.     

Basingstoke Canal Hydrological Study: A Water Balance

The paper describes a hydrological study to investigate causes and potential solutions to a significant shortfall of water in the Basingstoke Canal. Canal inflows and losses were investigated under drought and average summer conditions, and spring yields, streams, piped drainage and pumped inflows were assessed. Losses included evaporation, seepage, weirs and lock usage. A computer model of the complete water balance in the canal was created for the 29 pounds^† for different levels of canal usage under three meteorological scenarios. This showed the balance of water stored, and in shortfall or excess, for each month over a year of operation for each pound.
Field investigations, which were carried out to verify the model and to refine preliminary estimates, included flow measurement, site investigation, groundwater level monitoring and water-quality testing. Recommendations were made on potential solutions to the problem of water shortage to achieve different levels of canal usage.     

The Use of Bone Charcoal in the Treatment of Rural Water Supplies

As a result of the Water Act of 1989 on the quality of water intended for human consumption, a number of small spring sources in Yorkshire were the subject of legal undertakings for remedial action to reduce the concentration of trihalomethanes in the distribution system. The trihalomethanes are formed when the colour in these waters, which is made up of predominantly fulvic and humic acids, is chlorinated. Therefore, in order to solve the problem of trihalomethanes in treated water, colour removal was necessary.
The paper describes the use of bone charcoal in slow sand filters to remove colour from rural water supplies. It covers six months pilot-plant work which was undertaken at Marsett water-treatment works (near Richmond), and the implementation at other plants within Yorkshire. low-rate filtration through bone charcoal ensured that the colour and trihalomethane concentrations in filtered water complied with EC standards, and the material coped well with rapid changes in raw water quality.     

Catalytic-Iron Filters for Effective and Low-Cost Treatment of Odorous Air

Several methods are available to remove hydrogen sulphide and volatile organic sulphides from ventilated air at sewage-treatment works. Some methods use alkaline and oxidative chemical scrubbers which may have high capital and operational costs. Other methods, such as bio-scrubbers and bio-filters, are used to biochemically oxidize sulphides. Biological scrubbers have the advantage of low operating costs, but their performance may be adversely affected by a high concentration of sulphide (> 100 ml/m³).
This paper describes the development of a novel, compact and low-cost 'rusty-iron' chemical filter which catalyzes the oxidation of hydrogen sulphide and organic sulphides to colloidal sulphur and water. Operating costs are relatively low, with no need for routine maintenance. Full-scale demonstration of the effectiveness of such filters was carried out over two years at several treatment works, the results of which are described and discussed together with details of capital costs. The overall percentage removal of hydrogen sulphide by compact multistage filters was found to exceed 95%, irrespective of inlet sulphide concentrations which varied from 34 to 500 ml/m³ (ppm).     

Derivation of an 88-Year Inflow Record for Talla Reservoir, Scotland, with Special Reference to Low Flows

A new synthetic inflow record for Talla Reservoir is derived from reservoir log data for the period 1907–94. The paper describes (i) the procedures which were adopted for the derivation, based upon a reservoir water balance, and (ii) the changes in contributing area. For the years since construction of an upstream reservoir in 1968, the summer analysis draws on data from a neighbouring catchment in order to provide analysis of a more homogeneous time series. The paper also discusses the possibility of undertaking similar work for other reservoirs, as a means of improving the knowledge of the past variability of UK water resources.     

Constructed Wetlands: A Strategy for Sustainable Wastewater Treatment at Small Treatment Works

The use of sub-surface flow constructed wetlands planted with Phragmites australis (the common reed) for the treatment of sewage is now widespread in the UK. The strategy for the use of reed-beds by a major UK water utility is reviewed in terms of whether the process (a) is appropriate within the rural environment, (b) offers satisfactory performance, and (c) meets criteria for sustainability. Its use for secondary, tertiary and storm-water treatment is considered. Odour generation has been a concern for secondary treatment reed-beds and has been measured and modelled to identify the required minimum distance to properties to eliminate any possibility of odour nuisance.     

Statutory Water Quality Objectives and River Water Quality

This paper summarizes some of the key issues which have been considered in developing proposals for a scheme of classifying river quality for the purpose of setting water quality objectives on a statutory basis. Particular attention is given to (a) the relationship between use-related and general water quality targets, (b) the role of biological assessment, and (c) the requirements of EC directives. Following widespread consultation on the proposals, it was concluded that a clear distinction should be made between sets of 'use classes'which would serve to set statutory water quality objectives (SWQOs) and a general quality assessment scheme which would be applied independently to assess trends and general progress over a period of time. Once the classification scheme has been established by the Secretaries of State through regulations, SWQOs will be introduced to river stretches in a number of 'pilot'catchments across England and Wales, following local consultation. The setting of SWQOs will provide the focus for local decision-making on what the appropriate level of water quality should be – taking into account the desired uses of rivers, what will be required to achieve them, and the associated costs.