For every million gallons of water used in the home and by industry, nearly a million gallons of contaminated water have to be disposed of into inland or tidal waters. In the early days of the settlement of New Zealand, pollution of natural waters did not present a great problem. The population was small and as there were few waterborne-sewage disposal systems, the bulk of the human wastes did not reach inland or tidal waters. As time went on the population increased considerably, urban communities grew into large towns, farming became more intensive, and secondary industries developed. This led to the introduction of waterborne-sewage disposal systems with outfalls into streams, rivers, harbours, or ocean waters, and to outfalls from industries discharging large quantities of liquid wastes, particularly those associated with primary industry, such as freezing works, dairy factories, woollen mills, and such like. The consequence was that more and more of the country's natural waters were polluted in varying degrees, and it became more difficult, for example, to locate uncontaminated sources for urban and industrial water supplies.
With the growth of the population, development has spread into areas which were formerly remote places. Whereas years ago discharges of polluting wastes from the more remote towns and rural industries did not cause offence to many, the increased use of the motor vehicle and improved roading has meant that areas which were once regarded as out of reach are now popular recreational areas where the quality of the water is of considerable concern to the public.
Under the Waters Pollution Act of 1953, the Pollution Advisory Council was established with the responsibility of preventing and abating water pollution throughout New Zealand. In 1963 regulations were made under this Act to permit the Council to classify inland and coastal waters according to their established or potential uses. These classifications and water uses are defined as follows:
| Fresh Water | Saline Water |
| A. Controlled upland catchments used for public water supplies | SA Waters used for shell-fishing. |
| B Streams in lowland areas used for public water supplies. | SB Waters used for public bathing. |
| C Waters used for public bathing. | SC Enclosed waters such as bays, harbours, and estuaries not used specifically for bathing or shellfishing. |
| D Waters used for agriculture, wildlife, fishing, etc. | SD Waters along open coasts not used specifically for bathing or shellfishing. |
Water quality standards have been laid down to protect these different uses in the receiving waters. Following the classification of any waters, all outfalls discharging polluting wastes into them must be registered and covered by a permit setting out the conditions under which discharge may be made. The permit also indicates the standard of treatment of the waste discharge which the Council considers necessary to maintain the prescribed standard in the receiving waters. It then becomes an offence to allow the discharge of a waste which is likely to cause the quality of the receiving water to vary outside this standard.
How best to deal with several million gallons a day of contaminated wastes is a huge and costly problem. A survey conducted in 1962–63 by the Ministry of Works indicated that about 85 per cent of the total population of the cities and boroughs (1,639,000) was served by a public waterborne system of sewage disposal. The proportion of this urban population provided with a generally satisfactory form of public-sewage disposal system was just less than 60 per cent, while a further 15 per cent was served by an unsatisfactory system.
The treatment of waterborne wastes can be broadly divided into two stages, viz., primary treatment and secondary treatment. In addition, a third stage or tertiary treatment process may be required in some circumstances to protect the uses made of the receiving waters. Primary treatment is concerned with the separation of as much as possible of the solids carried in suspension in the sewage, and their subsequent treatment and disposal. Solid matter is removed in preliminary treatment units, such as screens and grit traps, and by settlement in tanks which provide quiescent conditions for the removal of the solids by gravity settling. Secondary treatment involves the biological stabilisation of the liquid effluent, including the remaining suspended solids and the dissolved solids, from the primary treatment processes. Examples of secondary treatment are the trickling filter and activated sludge processes. The tertiary treatment or “polishing” of an effluent to remove any residual polluting matter or bacteriological contamination may be effected by the use of sand filtration or oxidation ponds, among other processes. In some cases two or more of these stages of treatment are carried out in a single treatment unit such as an oxidation pond or a Pasveer oxidation channel.
In 1961 Government approved the payment of subsidies to local authorities towards the cost of providing sewage treatment and disposal facilities for towns with populations of less than 20,000. Because the cost per head of population served by such facilities is generally higher in the case of small communities than for cities, the subsidies are available on a graduated scale, the highest subsidy (50 per cent) being payable to communities with a population of less than 1,000.
The following table gives the expenditure by local authorities on the construction and maintenance of sewage and drainage works over the period 1959 to 1964 compared with total expenditure for all works.
| Year | Total Annual Expenditure on All Works | Annual Expenditure on Sewage and Drainage | Percentage of Total on Sewage and Drainage |
| £ | £ | Per Cent | |
| 1959–60 | 66,197,345 | 9,321,075 | 14.0 |
| 1960–61 | 68,819,859 | 9,309,001 | 13.8 |
| 1961–62 | 75,876,809 | 9,659,133 | 12.8 |
| 1962–63 | 79,327,917 | 10,291,661 | 13.0 |
| 1963–64 | 74,578,179 | 3,599,579 | .. |
These figures were abstracted from the Local Authority Statistics published by the Department of Statistics. NOTE—Total expenditure on all works excludes that by special-purpose authorities apart from urban drainage authorities.
by Denis Anderson Ferrier, B.E.(HONS.), B.SC.(N.Z.), D.I.C. Civil Engineering Division, Ministry of Works, Wellington.