Many readers will have some knowledge of how nature works but many of those who press for treatment do so through lack of just this knowledge. This chapter also reviews basic principles of sewage treatment as the two subjects are closely related.
Organic wastes are biodegradable, meaning that nature can recycle them into the basic ingredients from which all organisms are made. If this were not so, life would have ceased long since. The end products, following such recycling, are therefore not a problem in our environment. The sea, for example, is designed to handle the excrement, urine and putrescing dead bodies from all the creatures that live in it and from the rotting of all the vegetable matter in quantities hugely greater than any waste man contributes. However, in certain lakes, and exceptionally in some other water bodies, these end products, or nutrients, can cause a build-up of unwanted organic growth, such as weeds. Usually, however, it is thought to be beneficial to discharge nutrients to the oceans, as on land with compost, as many believe the sea to be nutrient deficient.
Apart from that unusual situation, pollution from sewage, where it occurs, is caused during the process of change from wastes to nutrients, when its demand for oxygen exceeds the oxygenating capacity of the receiving waters. Thus a stream will be denuded of oxygen by an untreated discharge of sewage if it exceeds the stream's carrying capacity. All streams, however, have some capacity, the bigger and faster flowing the stream the greater being its assimilatory capacity for wastes. At some point further down our stream, when the organic matter has been sufficiently oxidized, pollution will cease. The stream will be polluted for the distance between.
A secondary plant simply mimics nature's processes but at a faster rate than would occur naturally. It may be mechanical or, for smaller areas, simply a series of lagoons. The discharge from such a plant will only have an oxygen demand of a tenth of the raw sewage in a typical case. The volume of liquid from a sewage treatment plant is fractionally less than the inflow and would usually be discharge to surface waters or to the sea. The balance is in the form of a sludge that must be disposed of on land.
Thus, on the same stream, the discharge of a fine-screened sewage from a village will have the same effect as the discharge of a secondary treated effluent from a town ten times bigger.
Treatment does not convert sewage to water and it would serve no useful purpose to try and do so. It only carries out nature's own processes in a confined environment, to the stage by which it can handle the remaining oxygen demand without pollution being caused.
In confined waters receiving large volumes of waste, such as the Alberni canal, oxygen demand can be important. However, our long outfalls produce no net oxygen demand. Therefore to control this demand is pointless.
Additionally, pollution may be caused by the deposition of readily settleable or floating solids. Primary treatment removes both of these. The fine screens on our long outfalls do the same. The removal of these solids will also remove the oxygen demand that is associated with them, thus reducing the overall demand by about a third.
These levels of treatment will also reduce pathogens and enable chlorination to be more effective in reducing them further where this is necessary.
Such pathogens must be controlled for sea discharges and that can be done in two ways. One is to discharge the waste at such a depth and so far from shore that the pathogens cannot impact on health. Salt water is not a natural medium for human pathogens and their concentration at shoreline is diminished not only by dilution but by a phenomenon known as 'die-off'. These two factors are used in determining the length of outfall that is needed. The other is to construct treatment facilities and chlorinate the discharge. Chlorine, however, is poisonous. Accordingly, chlorination will probably produce an effluent more toxic than the original raw sewage and is less reliable than the long outfall technique.
Sewage may also contain non-biodegradable wastes, which nature is not designed to handle and which may damage or kill organisms. The wastes with such a potential may for our purposes be defined as toxic. It is essential to understand that primary and secondary treatment processes are not designed to overcome toxicity. They may do so to some extent depending on circumstances. On the other hand, too much toxicity in sewage will kill the organisms on which the treatment process depends with catastrophic results. In large measure these plants concentrate any toxic material and transfer it from the liquid fraction to the sludge.
Whether or not a substance is toxic, in fact, depends on its
concentration. As an example, man needs all kinds of substances
in his body that in larger concentrations are toxic. These
include arsenic, copper, molybdenum and a host of others. In the
case of chromium, primitive man had five times the amount in his
body that modern man has
Sea water also contains all these elements; chromium, for example, at about one fiftieth of the concentration in our blood and arsenic at about one thirtieth.
Man's blood also contains elements that are not known to be beneficial or are known to be harmful, some in minute proportions. Once again sea water contains all these elements, including, for example, lead, mercury and uranium, and has always done so.
It is not possible to pollute the broad oceans from the addition of such toxicants as are contained in domestic sewage. They have no impact and that will always remain so. As with discharges to streams and rivers, pollution of the sea from sewage can only be caused in a localized area, where dilution is inadequate or where a build-up occurs in bottom sediments.
The question of what is toxic and what is not therefore depends on the circumstances.
A further question is how significant toxic wastes in domestic sewage are liable to be, compared with other sources of toxicants. Such other sources of liquid wastes are those from industry and from storm sewers, namely those that carry rain wash from roads, roofs and yards.
Many industrial discharges are biodegradable but overall industry produces far more toxic wastes than do municipalities. They must be given treatment designed to suit each case. There is broad agreement that such wastes must be kept out of municipal sewers unless they have received adequate pretreatment; that is to say specific treatment at the plant site. That policy is in place in B.C. and its implementation well advanced, as opposed to numerous such discharges in less advanced and in third world countries.
In the general case, storm sewers are also likely to carry more toxicants than domestic sewage. All yard washings, all particulate matter washed out of the air onto roofs during rain, most domestic toxicants (a householder puts paint down the road gully rather than his toilet), malfunctioning septic tank effluent, and excess run off from herbicides and pesticides to road-side ditches, etc.
The only toxicants not already encompassed in the paragraphs above are houshold wastes that might be put down the sink or toilet. Some of these would be poisonous if imbibed at the concentrations used in households but are not in fact toxic after being diluted with the other liquids in the sewer. We need more information about these but they are certainly miniscule in impact compared with the other sources of toxicants that have been mentioned. If research ever shows that some are significant, they must not be made availabe for sale to the public. It is probably unrealistic to imagine that toxicants in such minute amounts will be removed at any kind of treatment plant. Indeed, toxicants in any significant proportion must be kept out of sewers because they destroy the biological organisms in the plant that bring about treatment.
Last to mention is tertiary treatment. That constitutes any kind of treatment beyond secondary. The plant at Kelowna, for example, is designed to remove nutrients and is therefore designated a tertiary plant. Where a discharge must be made to a stream affording little dilution, facilities may be added to lower the oxygen demand by more than a secondary plant will achieve. Typically in such cases, the demand may be reduced to one thirtieth of that of the incoming sewage, as compared with one tenth for a secondary plant and that constitues another example of tertiary treatment.