A British water pollution control -~
Regional boards, joint treatment, and unique inspectorate system are key features of U.K. program
204 Environmental Science 61 Technology
t a time when most industrial nations are seeking to control the pollution of inland waters by stricter laws or new legislation, Britain can claim that many of its rivers are now in a better condition than they were 20 years ago. Fish are returning to rivers that, not long ago, were devoid of such life; rivers once thought suitable only for use as industrial sewers are showing signs of improvement; and even rivers bearing a high proportion of purified sewage and industrial effluent now can be regarded as actual or potential sources of public water supply. These results have been achieved by the use of many types of purification plants in order to produce effluents to render them fit for discharge-mostly to public sewers, but also directly to rivers. It is doubtful whether this improved situation could have been developed during a period when two world wars halted expenditure on waste water
feature Samuel H. Jenkins Executive editor, Water Research
Oxford,England treatment, and at a time of rising population and mounting water consumption by industry, had it not been for a pollution control policy that took the form of a series of parliamentary acts, and the creation of new types of pollution control agencies. An understanding of this legislation and the functions of the agencies is necessary to appreciate why local authorities and manufacturers are pursuing a vigorous policy of waste water treatment. I n i t i a l legislation
After nearly a century of river pollution, caused to a large extent by industrial effluents, it was realized that such effluents were best dealt with by treatment in admixture with domestic sewage. In 1937, the Public Health (Drainage of Trade Premises) Act gave manufacturers in England and Wales the right to make use of public sewers for this purpose. The municipality owning the sewer was given the power to set conditions under which the discharge could take place, with the proviso that disputes were to be settled by the appropriate government minister. This act brought immediate benefits, but its full effect was not realized until after World War 11. In the River Board Act of 1948, the 1600 or so public authorities in England and Wales that inherited the duty of preventing pollution were replaced by 37 River Boards. These organizations consisted mostly of elected representatives nominated by local authorities in the area of each board. The catchment area of a river or group of rivers was mainly the area of administration, and the purpose of the boards was to implement existing laws on pollution prevention. The Rivers (Prevention of Pollution) Act (1951) extended the powers of the boards to require them to control all new discharges to rivers and altered outlets. Although boards were given discretionary powers in the standards of quality that could be set, generally, they required effluent standards with a maximum biochemical oxygen demand (BOD) of 20 mg./l. and a suspended solids limit of 30 mg./l. The river authorities have been
advised that, whenever they request higher standards, they should show that circumstances justify these demands. Standards not exceeding a BOD of 10 mg./l., suspended solids of 10 mg./l., and ammonia nitrogen of 10 mg./l. are becoming common. The legal interpretation of a consent to discharge is that compliance with standards must be observed at all times, even when the river authority takes a spot sample without giving notice of its intention. Thus, the average quality of the effluent obtained must always be much higher than the quality permitted in the consent. This differs from the policy adopted by many pollution control agencies where such control policy is based on the percentage of the organic impurity that must be removed by waste water treatment. It may be useful to remember that, in order to attain the BOD and suspended solids standards of 20 and 30 mg./l., local authorities have to remove approximately 96% of the impurity in the sewage they treat. Recent requirement
A recent requirement states that the ammonia nitrogen in sewage effluents must not exceed 10 mg./l. This means that a treatment plant must be designed to give complete purification to three times the dry weather sewage flow, and that 60-70% of the ammonia nitrogen must be oxidized at all times. Tanks with short retention periods, even with intense aeration, cannot meet these requirements because the nitrifying bacteria require a longer time to grow-or to oxidize the ammoniathan is provided in tanks of less than two to four hours capacity (calculated on the sewage flow, under ideal conditions of aeration, temperature. and sludge concentration) . Consequently. lightly loaded trickling filters are regarded favorably by river authorities seeking to achieve well-oxidized, nitrified effluents that will make the minimum oxygen demand on a river. Nitrified effluents have been obtained by combination treatment, such as a high-rate activated sludge plant, followed by distribution of the effluent on to a nitrifying trickling filter con-
taining a small grade medium. River control agencies regard nitrification as desirable because of the high oxygen demand of ammonia nitrogen, the toxicity of NH, to fish under certain circumstances, the reduction in chlorine demand, and the oxygen reserves present in nitrates. The 1951 Act gave river authorities control over discharges made after 1951, but an Act of 1961 extended this control to pre-1951 discharges. It was administratively impossible for the authorities to deal with the tens of thousands of such discharges within some of the areas?so no time limit was set within which pre-1951 discharges had to be handled. Positive approach
To many individuals and organizations, the term “pollution control” or “prevention” strikes a negative note. A more positive attitude toward the subject of water management has been adopted in the Water Resources Act of 1963 for England and Wales. Under this act, newly created river authorities ( i n place of the river boards) became responsible for water conservation by controlling the pollution of surface and underground waters. By charging a license fee for permission to abstract such water, and an additional sum graded according to the quality of water and its volume and purpose stated in the license, the river authorities are expected to become self-supporting. Only by detailed control and continuous pressure on discharges to comply with legal requirements given in the consent to discharge, can improvements be made in the quality of river water. It is important to ensure that industrial wastes received by public sewers d o not prevent treatment of sewage to whatever standard is required by the river authorities. The fact that the effluents from municipal sewage treatment plants must meet the requirements of the river authorities, and th.: law that requires municipalities to receive industrial waste, has compelled local authorities to exercise their powers to control the discharge of these wiistes to public sewers. Volume 4, Number 3, March 1970 205
Such control takes several forms. Usually, the total volume discharged in any one day, the rate of discharge, and the composition are controlled. This may call for some pretreatment of the industrial waste before it is allowed to enter the sewer. The simplest form of pretreatment needed may be a balancing tank for equalizing the rate of discharge or its composition. More usual requirements are the removal of excessive amounts of suspended matter or screenable material from abattoir wastes, or vegetable fiber, paperpulp, rags, etc. The removal of oil, down to limits of 10 mg./l., necessitates the use of oil separation tanks, usually after the addition of flocculants at a controlled pH. Metal finishing wastes
The metal finishing industry has recognized that treatment of mixtures of effluents containing acids, alkalis, cyanides, and compounds of cadmium, chromium, copper, nickel, and zinc in high dilution is needlessly expensive. The pretreatment tanks required are very large and the cost of the chemicals needed is prohibitive. Much thought has been applied to the development of metal finishing plants that: Make economical use of water. * Minimize the production of unusable effluent. Segregate effluents into those that may be treated separately and those that may he discharged without pretreatment. Permit the use of plastic pipes carried on trays mounted on the walls for the conveyance of effluents, instead of underground drains that are expensive to lay or replace. In one plant, overground drainage has saved an estimated $60,000 in effluent treatment plant construction costs. Some three miles of plastic pipework is used at the plant-a RollsRoyce engine factory in England. The strong wastes are diluted automatically with sufficient weak swill water to allow continuous and automatic treatment; cyanides are treated at a pH of more than 10 with chlorine gas; chromates are treated at pH 2-3 with sulfur dioxide gas; then the two treated liquors mix and are neutralized automatically, either with milk of lime or
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206 Environmental Science & Technology
Nottingham. Throughput capacity of Stoke Bardolph works now is being doubled
sulfuric acid. Finally, the effluent is settled for four hours in an upwardflow tank of 48,000 gallons in order to settle out the precipitated metal hydroxides. Only acid effluents which are neutralized may bypass the settling tank. All the sludge is dewatered by filtration through rotary vacuum filters. Other new ideas on pretreatment that have been put into practice recently at a Rolls-Royce aircraft engine factory in Scotland involve the treatment of spent plating and similar liquors that are so concentrated that the precipitates formed are sufficiently thickened to enable settlement tanks to be bypassed. Plants of this kind are designed with the intention of using water sparingly, partly hecause a high degree of purification is necessary to meet the standards required to discharge the effluent to public sewers, partly because of the mounting costs of water, and also because payment has to be made for discharges of industrial waste water to sewers. Industrial reuse
Water reuse is practiced on an increasing scale in industry. Cooling water either is recycled through lagoons or tanks, or through evaporative coolers. In the steel industry-where,
for example, 180 tons of water are recirculated for every ton of steel produced-the net water consumption is only five tons per ton of steel. In one plant, 25% of the fresh water intake becomes sewage effluent. Another example of the pretreatment necessary to enable a relatively small sewage works to accept the effluent from a large food factory is the segregation of effluents into strong and weak liquors and the introduction of rigid control measures to reduce water consumption. Suspended matter is removed from strong process liquors by centrifuging and vacuum filtration; all the strong liquors are concentrated in multiple effect evaporators; and solids and evaporated liquors are incinerated in a factory boiler. In fact, incineration is becoming recognized as an ideal solution to the disposal of strong organic wastes nf small volume. In order to ensure that all manufacturers comply with the legal consent they receive from the municipality, local authorities in Britain maintain an inspectorate service. Trained and qualified inspectors have powers of entry to trade premises for the purpose of taking samples of effluent passing to the public sewer. The manufac-
turer must provide a suitable sampling chamber for this purpose. On the basis of the sample analysis, the local authority may calculate the charge to be made for the treatment of the effluent and also determine whether it is of acceptable quality. If it is not up to standard, the manufacturer may have to install a pretreatment plant. When cooperation and/or persuasion fail to bring about improvements, the local authority can take legal action on the basis of a legal sampling procedure. These samples are divided into three parts in the presence of a responsible official of the manufacturer. One portion is to be analyzed, one handed to the manufacturer, and one retained for examination by the court in case the result of analysis is disputed. The procedure before a magistrates court is simple and effective, since a fine may be imposed for each day on which the offense (failure to comply with the consent condition) is committed. There is little room for procrastination or defense, since the action taken rests only on proving that the effluent comes from the manufacturer’s premises and that its composition shows that it is outside the limits of the consent.
Using the consent procedure, it has been practicable, over a period of years, to control the discharge of effluents from 2000 premises in an area with a high concentration of metal finishing plants. About 400 pretreatment plants have been installed for the removal of oils, fats, screenings, suspended solids, alkalis, acids, solvents, nonferrous metals, iron salts, entrails, and organic substances. Without such control measures, it would have proved extremely difficult-perhaps impossible because of the expense-to purify sewage containing 60% of industrial wastes to the standards required by the river authority responsible for water management. National policy
The knowledge that local authorities are prepared to accept industrial effluents allows the river authorities to insist upon manufacturers complying with stringent standards if they wish to discharge to a river. National policy also requires that local authorities make adequate provision for the conveyance and treatment of all industrial effluent in the district at their sewage works when extensions are being planned. If the
scheme does not make adequate provision for such effluents, the omission usually draws attention at the public inquiry held by the appropriate government department to sanction financing for the scheme. Official action even may encourage joint schemes for the treatment of sewage and industrial wastes. The latest development in encouraging joint treatment of industrial effluents at municipal plants is to ensure that local industry is officially represented at the public inquiries. Industry then has an opportunity of requesting, in public, that adequate provision is made for treating all the industrial waste water in the district. Industry is required to pay for the treatment of its effluents by the local authority. The general method is a sewage or conveyance charge based upon the volume of the effluent. This principle also is used to assess the charge for sedimentation or for treatment in a plant which is designed on hydraulic considerations. A charge is made for biological oxidation and sludge treatment, depending on the proportion of the plant occupied by the industrial effluent in purifying the mixture of sewage and effluent. The charge may he made on a volumetric Volume 4, Number 3, March 1970 207
Scotland. Activated sludge unit being built at Kelso basis and inclusive of loan repayment and operating costs, or the manufacturer may make an outright capital payment and thereafter pay the operating costs. These charges may he varied from time to time to account for rising costs or charges on new capital expenditures. Sewage treatment plants
The emphasis on effluents with a of less than 20 mg./l., and additional standards for metals, cyanides, phenols, and oils, explains why so many plants in Britain appear to he nnderloaded by comparison with loadings common in the US. A further reason for the generous capacity is the insistence that the maximum proportion of storm water also must he purified in order to minimize pollution. For the past 50 years, trunk sewers in Britain have been designed to convey six times the dry weather flow (d.w.f.). At the sewage treatment plant, screens and grit tanks are designed to handle this fiow, and sedimentation tanks, oxidation plant, and secondary tanks are designed to give complete purification to 3 d.w.f. The remaining 3 d.w.f spills over into tanks, usually of at least six hours’ d.w.f. capacity, with facilities for sludge removal and for pumping hack BOD
208 Environmental Science & Technology
Birmingham. Aeration tank at Coleshill works
for full treatment the aqueous contents as soou as capacity is available to receive it. Overiiows of settled storm water sewage to the watercourse are not permitted until full treatment is being given to 3 d.w.f. In small- or medium-sized works for populations of up to 20,000, the usual method of sewage treatment is sedimentation-biological oxidation in trickling filters, followed by sludge drying on open-air beds with or without sludge digestion. Innovations in sedimentation are in the direction of simpler design and cost saving by the use of tanks with a floor slope of 7S0,and a single link chain dragged around the floor by the operation of a centrally located electric motor. Sludge with 10% dry solid matter can be moved in this way. Efficient operation of trickling filters usually is obtained by recirculation or by alternating double filtration, although, if a minimum ammonia nitrogen standard is required, it may he necessary to restrict both the hydraulic and the organic loading. The use of power driven distributors, controlled at a relatively slow rate of revolution, is contrary to the American practice of rapidly moving distributors. But large-scale experiments over many years have proved that excessive surface growth on a filter
can seriously lower its efficiency, and that the way to control such growth is to restrict the rate of travel of the distributor, usually to within 5-20 minutes per revolution. Plastic filter media
Plastic filter media of various makes are linding increasing application, mainly for the treatment of industrial wastes. In overloaded sewage treatment plants, such filter media have the advantage of being able to remove substantial amounts of organic matter from poorly settled or even screened sewage. Considerable experimental work is being done on the use of plastics media. Another simplification in filter design is in the construction of filter walls. For years it was maintained that filters had to be built above ground level so that air could pass upwards from basal ports. Because of this, few filters survived their useful span of life without requiring major repairs to walls, due to the inability of the walls to contract to their original length in winter. Experience in recently constructed plants has shown that, if the base of the filter wall is never waterlogged and the effluent flows freely, the walls can be submerged completely or covered by an earth bank. Waterlogging of
Samuel H. Jenkins is executive editor o f Water Research, and consultant in water pollution control. Previously, he was president, Znstitute of Water Po/lution Control. He received his B S c . and M A . f r o m the University of Manchester (England), and continued studies at London University. Jenkins started his career in the texfile industry, spent time in beet sugar and mill processing research, and joined the Birmingham, Tame, and Rea District Drainage Board in 1938.
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filters to control flies is quite unnecessary, since this can be done effectively with DDT or henezene hexachloride. Trickling filters and secondary tanks cannot always be relied upon to reduce suspended solids to the limit set by a river authority. Since these solids may account for 2 5 5 0 % of the BOD of an effluent, tertiary treatment to remove them is becoming common. Treatment on specially laid grassland is effective at dosage rates of up to 300,000 gallons per acre per day. Where lack of space makes such methods impossible, treatment in gravity sand filters or microstainers is practiced. Another, less expensive method is to cause the secondary tank effluent to flow upwards through a 9-12 inch filter of coarse gravel laid on a perforated metal grid, with a covering of finer gravel. Almost all of the upwards flow filter is submerged in the effluent, so that the device may be located in the tank. Much greater attention will be given in the future to the removal of suspended matter from effluents, because recent work has shown that such matter makes a greater demand on the oxygen reserves of a river than was formerly thought to he the case. In Britain, the activated sludge process was reserved mostly for treatment
of large-scale sewage until the introduction of reliable mechanical aerators effective in the small plants. Diffused air, on the whole, is used for large plants, hut mechanical aerators are seen in plants treating up to 72 million gallons per day. Air generally is diffused through vitrified domes attached to pipes laid on the aeration tank floor. Plant-scale experiment has led to the development of a simple activated sludge process. The maximum degree of mixing and the oxygen demand is met at all times by means of a single pass, 60 foot aeration tank, with a submerged inlet the full width of the tank. The air supply is 3.1 cubic feet per gallon, one half of which is provided in the first one third of the tank. One plant of this type is partly in operation for the treatment of 20 million gallons per day and another, under construction, will treat 48 million gallons per day. Both plants are designed to achieve at least 75% nitrification of the ammonia nitrogen. For the treatment of sewage from small communities, considerable use is being made of extended aeration systems. These include oxidation (Pasveer) ditches, often with a sludge separation tank to allow continuous aeration. The same standards required by river authorities of large plants apply to such small plants, and, consequently, an additional stage of treatment may he necessary to remove excess, well oxidized, suspended solid matter sometimes present in effluents from extended aeration plants. Irrigation over grassland is the preferred method of treatment for this purpose, due to its reliability and simplicity. Because of the large capital cost of new sewage treatment works, there is constant questioning of cost-henefit relationships, and consideration of alternatives to some of the traditional unit processes that are employed. At present, biological treatment, in comparison with complete treatment by flocculation, ion exchange, sand filtration, and reverse osmosis, remains the cheapest method of removing organic matter in the low and variable concentration in which it occurs in sewage. Thus, the most suitable application for the newer processes is to allow a further stage of purification
than can he achieved by the existing unit processes. Sludge disposal
Sludge disposal is an acute problem in Britain. With scarcity of land and objections to exposed sludge lagoons, the search for alternate means of sludge disposal has become urgent. A succession of wet summers in Britain has accelerated the move toward mechanical sludge dewatering. Vacuum filtration is on the increase, especially in medium-sized works, and great interest also is being shown in the use of plate pressure filters of new design, mainly because they are compact and produce a thick sludgecake with less than 70% moisture. Sludge pressing without any coagulant is successful if the sludge first is cooked at a fairly high temperature and put under pressure for a brief period to improve filtration properties. The full potential 01 this process may be realized when a suitable method has been devised to deal with the organic matter in the sludge liquor. With growing interest in sludge incineration, there is likely to be a trend toward continuous removal and dewatering of fresh sludge hy mechanical methods with the aid of efficient nonbulky flocculants. Such sludge has a high calorific value, and can he incinerated to a slagging ash with only 10% of the volume of the ash in powder form. However, this forecast could he upset easily as the result of some fundamental work being done on the physical properties of sludge. The present position in Britain can be summarized as one in which rivers are beginning to improve in quality as a result of the extension of sewage treatment facilities. A major improvement is attributable to encouraging the discharge of industrial waste water to municipal sewers. This policy has been possible only by providing municipal authorities with full powers to control all such discharges so that, even when the proportion Of industrial effluent is high, the mixture of sewage and industrial effluent still can he treated to the required standards, Cooperation between industry, local authorities, and the river authorities is essential, nevertheless, these authorities still need legal owers to set and enforce their standards. Volume 4, Number 3, March 1910 209
