Nov 1993

Increased urban development and requirements for tighter control of the quality of effluents discharged into rivers, estuaries and coastal waters has led to the need to operate wastewater treatment works close to residential and commercial areas. Odour from treatment works is often considered to be a significant nuisance. Enclosure of the works enables the odours emitted to be reduced.

Operations at wastewater treatment works give rise to potentially hazardous emissions, for example:

• flammable gases/vapours
• toxic substances, most notably hydrogen sulphide
• microbial aerosols

Conventional design practice for open air treatment works allows natural atmospheric dispersion processes to dilute harmful substances to safe levels. In enclosed works, the natural atmospheric dispersion processes can no longer be relied upon, and their design should address explicitly how the exposure of workers to these hazards is to be avoided.

The general approach to controlling these emissions in enclosed works is:

• control the quantities of toxic or flammable materials discharged into the sewers;
• discourage the formation of toxic gases within the sewers;
• isolate the end of the sewer from the treatment works building;
• select unit operations and operating practices that minimise the emissions within the treatment works;
• prevent emissions from the works by enclosure and operation at negative pressure;
• contain emission sources wherever possible;
• apply local ventilation measures wherever possible;
• restrict the spread of emissions through the works by compartmentation;
• apply general ventilation to dilute emissions to acceptable levels;
• treat ventilation air before discharge;
• disperse treated air to dilute it to acceptable concentrations outside the plant.

Control measures for flammable gases and vapours are particularly important at the influent works and wherever flammable gases are generated, for example during anaerobic digestion. Particular attention should be taken to exclude flammable liquids and vapours from the works. It is recommended that unit processes generating flammable gases should not be carried out in enclosed works. The risk of explosion from the ignition of flammable gases and vapours can be reduced by eliminating potential sources of ignition and by ventilation. Measures such as explosion venting should be taken to protect workers and passers-by and the integrity of the building wherever explosible concentrations of flammable gases and vapours are likely to occur.

Toxic gases and vapours, emitted during wastewater treatment, are present as a result of toxic discharges to the sewer and as a result of chemical and biological processes in the sewer and at the treatment works. Hydrogen sulphide is the most important toxic gas consideration. The emission of hydrogen sulphide can be reduced by containing the most significant sources and by local ventilation measures. However, general ventilation is usually the major control method. The design of general ventilation systems requires estimates to be made of the rates of emission from specific unit processes. Accurate estimates of the rates of emission are difficult to make. However, useful estimates can be made on the basis of equilibrium calculations and using the Reynold's analogy to estimate mass transfer coefficients.

Microbial aerosols are released from many wastewater treatment processes particularly those that generate spray. Activated sludge tanks, for example, are often significant sources of biological aerosols. It is uncertain whether the aerosols generated at open-air wastewater treatment works are a hazard to workers. Bearing in mind the potential for enhanced operator exposure within confined conditions in enclosed works, it will be prudent to minimise or even prevent the release of aerosols. It is therefore recommended that significant sources of aerosols are covered wherever practicable. However, low energy processes such as sedimentation are not likely to generate significant quantities of aerosols.

Enclosing wastewater treatment works can also cause problems associated with condensation, corrosion, temperature control and odour control. These problems are considerably alleviated by measures used to control toxic and flammable gases and bioaerosols. In particular, general ventilation to dilute hydrogen sulphide emissions to acceptable levels often ensures that further measures to control these problems are unnecessary. However, in some circumstances additional ventilation may be required. Useful estimates of emission rates, usually sufficient for the design of ventilation systems, can be made on the basis of equilibrium calculations and using Reynold's analogy to estimate transfer coefficients.

Noise from ventilation equipment and other equipment within the works can cause a nuisance to local residents and commercial premises. If uncontrolled, the noise generated within the works may be sufficient to damage workers hearing. Estimates of the noise levels inside and outside the works can be made and measures taken to reduce the noise where necessary.

Wastewater treatment works are often situated on relatively low-lying ground in order to encourage the flow of water to the plant through the sewers under gravity. There is therefore a risk of the enclosed plant becoming inundated with water in the event of high flood water levels. The trend to build enclosed works underground, often partially below the water table, increases the risk of flooding from storm water. An analysis should be made of flood water levels, peak hourly wastewater flowrates and the hydraulic capacity of the works and the effluent pumping systems. Complex control systems may be required to control water levels and flowrates through the works: formal studies of the reliability of these systems should be carried out.

Without careful design and planning a fire in an enclosed works could spread rapidly, endangering the lives of the workers employed there, destroying the treatment plant and damaging the structure of the works. The spread of fire and smoke may be prevented by :

• selection of surface materials with low rates of surface spread of flame and low rates of heat production;
• choosing elements of the structure with specified minimum periods of fire resistance;
• the use of non-combustible materials wherever possible;
• the compartmentation of buildings into units of restricted floor area and capacity;
• the protection of openings from doors, pipes, water channels, ventilation ducts, flues etc. between compartments;
• restricting the spread of fire through hidden voids in the building;
• specifying construction methods for external walls and roofs and the separation between buildings;
• special measures to protect spaces connecting compartments.

Means of escape in case of fire should be clearly defined. Distances of travel to the final exit point should be limited. A fire defence plan should be drawn up covering the detection of fire and all the arrangements needed to attend and attack the fire.

Enclosed treatment works have little access to natural light and it is usually necessary to supply artificial light in order to maintain safe conditions of work. Emergency lighting should be provided to allow workers to leave the building safely in the event of a power failure or other emergency.

The advice given in this guide will enable the designer and operator of enclosed wastewater treatment works to:

• identify the problems resulting from enclosing operations;
• indicate how these problems can be quantified in relation to design parameters:
• describe measures for the control and monitoring of these problems.

Relatively few enclosed wastewater treatment works have been built. The advice given in this guide, therefore draws on information related to a broad range of process industries as well as the water treatment industry. Experience gained in the design of new plants using the advice given will demonstrate where the advice is adequate and where shortcomings exist.

Copies of this report are available from FWR, price £40.00, less 20% for Members.