Section 8 of which provides that local authorities and other parties (Part 1)

Section 8 of which provides that local authorities and other parties shall use the best practicable and available means to render harm- less the sewage discharged into streams and rivers, while the Rivers (Prevention of Pollution) Act, 1951, gives River Boards the right to control the discharge of trade effluents into streams, and insists on a minimum standard of purity before the effluent of sewage purification plant and apparatus or plant for dealing with liquid trade waste can be discharged into streams and rivers. Need for Purification. It is necessary, therefore, to see that the sewage, before being discharged into streams, is purified and not clarified only; that is to say, not only shall the matters in suspen- sion be removed, but the organic impurities also, so that secondary decomposition shall not set up after the effluent water has com- mingled with the water in the stream. Sewage is a very complex substance, its composition varying tremendously. It is obviously the fact that there must be a great difference, for example, in the composition of the sewage from a purely residential district and that from a large manufacturing town, owing to the sewers receiving the waste liquids from the trades and manufactures, which in themselves, again, differ in nature in one town from those in another. Composition of Sewage. It is difficult, therefore, to give any- thing very definite as to the chemical composition of sewage SEWAGE DISPOSAL 885 The Rivers Pollution Commissioners, in 1876, published a table of compositions of sewage for both water-closet towns and privy midden towns, but the latter are so rapidly becoming obsolete that they hardly call for notice now. An abstract of the table referred to is given below as regards water-closet towns, i.e. towns discharging principally ordinary domestic sewage into the sewers: AVERAGE COMPOSITION OF SEWAGE Suspended Matters Mineral | Organic} Total Total Solid Matters in Solution Organic Carbon Organic Nitrogen Ammonia Total Combined Nitrogen Chlorine In Parts per 100,000 72-2 | 4-696 | 2-205 | 6-703 | 7-728 | 10-66 In Grains per Gallon | 50-54 | 3-287 1-543 “002 5-410 | 7-462 | 16-926 81-283 the remainder being water. Since the date when the above-mentioned table was compiled it has been discovered that a part of the solid matters formerly supposed to be in solution is in reality in a state intermediate between solution and suspension. These matters are known as “colloids” and they are particles of a jelly-like character; the purification of the colloidal matters has been one of the most difficult problems in sewage treatment. Bacteria. Sewage is highly charged with various kinds of bacteria, some of which, as we shall see later, are made use of in certain processes of purification. It is therefore well to consider, at the outset, what bacteria really are. Bacteria are minute vegetable growths, varying in size from about one-fifteen-thousandth to one-twenty-five-thousahdth of an inch in diameter; they increase usually by division, occasionally by spore formation; their multiplication is exceedingly rapid and is interfered with by cold. Moisture is necessary for their success- ful working. 13—D.s. 886 SEWAGE DISPOSAL Classes of Bacteria. They fall into two classes, namely, the © parasitic, needing a living host, and the saprophytic, living on — dead matter, but some exist indifferently as both parasites and — saprophytes. We have to deal with the saprophytic organisms, © which are subdivided into anaerobic, living without air, and aerobie — bacteria, living with air. | Crude sewage contains enormous quantities of anaerobic bacteria — and relatively very few aerobic, but it is possible largely to destroy — the former and to cause the latter to multiply, by prolonged aeration - of the sewage. The effect of anaerobic bacteria is to cause the — sewage to decompose, in which process part of the organic solids will be liquefied and gasified. The effect of aerobic bacteria is to enable the organic matters to combine with the oxygen present in the sewage and so to form harmless compounds, such as nitrates. The quantity of nitrates present in a sewage effluent is indeed an indication of the amount of purification by oxidation that has occurred in the treatment. It is, however, no indication of whether the effluent is fit to be discharged into a stream, for its fitness depends not on how much purification has been done, but on how much is left undone. A good criterion of this is the amount of oxygen which will be absorbed by a sample in a certain standard time, from a standard solution of potassium permanganate. It is difficult to define the meaning of the word “harmless” as used in Section 8 of the Rivers Pollution Prevention Act, 1876. The old Local Government Board was often asked to prescribe a standard of purity for sewage effluents. The Board, however, always refused to do so, on the ground that each case should be dealt with on its own particular merits. Investigations into Disposal 50 Years Back. In 1898 a Royal Commission was appointed to investigate the whole question of sewage disposal, and this Commission issued many reports. The eighth, issued at the end of the year 1912, deals with the question of standards of purity of effluents. A brief summary of the conclusions of the Commission on this point is as follows: “The law should be altered, so that a person, discharging sewage matter into a stream, shall not be deemed to have committed an offence under the Rivers Pollution Prevention Act, 1876, if the sewage matter is discharged in a form which satisfies the require- ments of the prescribed standard, the standard being either the general standard, or a special standard which shall be higher or | lower than the general standard, as local circumstances require or permit. SEWAGE DISPOSAL B87 “An effluent, in order to comply with the general standard, must not contain, as discharged, more than three parts per 100,000. of suspended matter and, with its suspended matters included, must not take up, at 65° F., more than two parts per 100,000 of dissolved oxygen in five days. This general standard should be prescribed either by Statute or by Order of a Central Authority, and should be subject to modifications by that authority after an interval of not less than ten years. “In fixing any special standard, the dilution afforded by the stream is the chief factor to be considered. If the dilution is very low, it may be necessary for the Central Authority, either on its own initiative or on application of the Rivers Board, to prescribe a specially stringent standard, which should also remain in force for a period of not less than ten years. “If the dilution is very great, the standard may, with the approval of the Central Authority, be relaxed or suspended altogether. Relaxed standards should be subject to revision at periods to be fixed by the Central Authority, and the periods should be shorter than those prescribed for the general or for the more stringent standards. “With a dilution of over 500 volumes, all tests might be dis- pensed with, and crude sewage discharged, subject to such con- ditions as to the provision of screens and detritus tanks as might appear necessary to the Central Authority.” Since that day, much research has taken place and quite a number of statutes (some of which will be referred to in Chapter XVI) have been added to our legislation. Prevention of River Pollution. The Rivers Boards Act recently passed divides the country into areas, each under the care of its own local River Board. The last Rivers (Prevention of Pollution) Act (1951), gives these river boards the duty of making by-laws and determining standards of purity of sewage effluent which may be discharged into the rivers in their several areas (sec. 5). These standards vary from area to area, so no national standard of purity exists and river boards are guided mainly by the character of the rivers in their charge. Insufficiently purified sewage effluent discharged into a river increases the proportion of organic matter, the number of bacteria and the number of parts of dissolved oxygen per 100,000 parts of water in the river. The bacteria and other micro-organisms (apart from their power to propagate disease if allowed to con- taminate drinking water), use for their living processes some of the oxygen held by the water. The amount of oxygen so utilised 388 SEWAGE DISPOSAL or absorbed is generally referred to as the “Biological Oxygen Demand” (or B.O.D.). A well-purified sewage effluent should give a B.O.D. value of 0-5 parts per 100,000 or less, and most sewage works managers try to maintain a standard of sewage effluent not exceeding 2 parts per 100,000, which, when taken into the stream of a river with a reasonable volume and velocity of flow, will dilute down to safe conditions for fish life and for the tiny animal or vegetable organisms which form the food of the fish. This standard of B.O.D. is also a good indication of the number of bacteria likely to be found in the effluent and so, indirectly, of its effect on the river as a source of domestic water supply. The whole question of sewage disposal is one in which there has been a vast amount of experimental work going on for years, and is one of the greatest importance to the public at large. The prompt and effectual removal of the excremental and other refuse from the midst of our communities is of the utmost importance, but no one system of disposal can be applied indiscriminately, and the system which is applicable to one district may not be by any means the best for another, differently situated or conditioned. Chief Classes of Disposal Methods. Broadly, the chief methods of treatment for the disposal of sewage may be classified so: 1. Discharge into the sea or a tidal estuary. 2. Land treatment (aerobic). 3. Some suitable combination of any of the following: (a) Sedimentation tanks, with or without the use of chemical precipitants (mechanical treatment). (b) Septic tanks or hydrolytic tanks (anaerobic treatment). (c) Activated sludge tanks (aerobic). (d) Contact beds or percolating filters (aerobic). Objectives to be Aimed at. No matter what treatment is adopted, the primary object is purification. Sewage works should be kept free from nuisance; the production of sludge, or sewage mud, must be avoided as far as possible, if there is difficulty in getting rid of it; and lastly, the expenditure, both capital and annual, should be kept as low as possible consistent with efficiency. Discharge into Sea or Tidal Estuary. In the case of towns on the sea-coast, discharge into the sea or a tidal estuary furnishes an efficient and economical means of disposal. Great care is necessary in selecting the position for the outfall, and careful observations of the nature of the prevailing currents should be made over a fairly large area. The observations are made with the help of floats, the directions taken being carefully recorded SEWAGE DISPOSAL 889 with the aid of a theodolite or prismatic compass, so that the course of the current may be plotted on a plan. The sewage will in most cases have to be discharged into favourable currents on the ebb tide, tanks, of course, being provided for storage during high tide. In a few exceptional cases it is possible to discharge the sewage at all states of the tide without causing a nuisance, in which cases storage tanks are unnecessary. Land Treatment. There are two methods of applying sewage to land for the purpose of purification, known as irrigation and filtration respectively. Irrigation. Irrigation is the system in which the sewage is made to flow over an area of land continuously for a certain time, the area being then rested while agricultural operations proceed. The land is not underdrained. This system is best carried out by using land which has a fall of about 1 in 100 and passing the sewage over it in a thin sheet. Land Filtration. On the other hand, filtration is generally carried out by using flat land or levelling the plots, underdraining them, enclosing them with earth walls or banks, flooding the area with sewage, and allowing it to soak away, thus passing alternate layers of sewage and air through the soil. In both systems ridges and furrows, as shown in Fig. 420, are sometimes used, the advan- tage being that the roots of the crops absorb the sewage without the crops themselves being fouled. The sewage carriers require to be carefully arranged and con- structed. The main carriers or channels can be of concrete, and the minor carriers can either be similarly constructed, or may consist of grips, or channels, just cut in the ground. Fig. 421 shows what is known as the catch-water system, applied to an area of rough and irregular surface. The minor carriers are care- fully formed along contours, at vertical intervals of about 2 feet 6 inches, as shown. The inlet to the main cafrier is controlled by a meter chamber, to measure the quantity applied at one time. The entrance to the minor carrier, at its junction with the main, is controlled by a small sluice. The land drains should not be of less diameter than 3 inches, and there is no advantage in putting them deeper than from 4 to 5 feet below the surface. A manhole is shown on the subsoil drain, for the purpose of sampling the effluent passing through the drains. Nature of Soil needed for Successful Land Filtration. A most important point in reference to the filtration treatment through land is the nature of the soil. A good loamy soil is the best, and clay or peat about the worst. Sand or gravel are of little use for SEWAGE DISPOSAL 390 S Jt-F NOILIFS Y PUL A A g-Y NOILOIS ‘Zp O-2 NOILIIS Yul SS NNAAAAARANRARAAAN i) NAANAAAAAAAAAARRRAN ISNOH TIVWS ¥ xOF SYILIA ONY SAHNVL INILVLId/IIAd AO SNOILOIS ONY NYTd LTO Sth, a4 LNIWLVYIAL ONYT YOY SNIVACAFON ONY SAxF/¢AVD FOVMIS YIGNVH) xTLIN JW SNiv¥d Wosans -~- #7/ALVI HINVAT LILA NVA -FINIAFAITY eee ae Oty NOILVALT/A ONVT YOF SMOXANA ONY SI9A/A NOILVANLYS SEWAGE DISPOSAL 891 the purpose, but peat and clay should only be used when there is a fair amount of vegetable soil above them, of at least 6 or 9 inches in depth. Good soils for purification purposes are alluvial drift and gravel, oolitic limestone, Bunter sandstone, magnesian lime- stone, chalk and old red sandstone. Importance of Top Soil. The bulk of the purification is effected in the first foot or so of depth, but it is necessary to have an under- lying soil of a porous nature in order to carry off the effluent. The action of an earth filter is partly mechanical, partly chemical and partly biological. The destruction of organic impurities in sewage is brought about by a process of active fermentation, termed nitrification, caused by aerobic bacteria, the organic matters being resolved into soluble nitrates, products having no smell, colour or injurious properties. Nitrification ceases if the treatment of the land with sewage is not conducted intermittently and, if irrigation or filtration is carried on without care and without regard to the scientific principles involved, there is a risk of the pollution of subsoil waters and streams. It is easy to see why a non-porous soil is unsuitable for sewage purification—although the sewage may be mechanically strained, the absence of free oxygen in the soil prevents purification. Given a suitable soil and good management, filtration through land will result in an effluent as pure as can be obtained by any other means; it must not be forgotten, however, that a sewage farm cannot be free from smell and should therefore not be near built-up areas. Interchange in Land Treatment. It is a very general practice now to work a sewage farm on the principle of one section being given up to irrigation and another to filtration, and to change them over once a year. This enables the land to be well worked up and aerated, especially if the ridge and furrow system is adopted, and the ridges and furrows levelled once a year. Italian rye grass is a favourite crop on sewage farms, the reason being that it grows so rapidly and closely that weeds are kept down—an important point in land treatment. Mangolds and other crops are also wn. - It will be obvious that, if the areas are interchangeable for both irrigation and filtration, the whole must be underdrained. That no hard-and-fast rule, or definite figure, can be given as to the area required for any particular case of land treatment will be evident from the following extract from the Fourth Report of the Royal Commission on Sewage Disposal: ““We doubt whether the most suitable kinds of soil, worked as a filtration farm, should 892 SEWAGE DISPOSAL be called upon to treat more than 30,000 to 60,000 gallons per acre per twenty-four hours at a given time (750 to 1500 people per acre), or more than 10,000 to 20,000 gallons per acre per twenty-four hours calculated on the total area of irrigation (250 to 500 people per acre). Soil not so well suited, worked as surface irrigation, or combined surface irrigation and filtration, 25 to 50 persons per acre.’ The sewage in the foregoing cases is assumed to have been settled in tanks before it is applied to land. It is impossible to lay down any hard-and-fast rule as regards the proper proportions between the area being irrigated at one time and the surplus resting or aerating, but the Commissioners sug- gested that four-fifths of a surface irrigation farm and two-thirds of a filtration farm, should be at rest. The Commissioners in effect also expressed their inability to say whether slow or rapid alternations of work and rest are advisable. The Commissioners laid much stress on the fact that the success or failure of a sewage farm lies largely with the