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Marine Scientists on Sustainable Management of Sewage Disposal

Sewage disposal requires a series of different approaches to different substances

Sewage is not a single contaminant but [...] a complex mixture containing pathogens, nutrients, suspended solids (SS), oxygen demanding substances, and many other contaminants – each with different environmental effects, and different responses to disposal and treatment.

It is therefore essential, in devising a sewage management strategy, to begin by identifying the environmental problems to be addressed and the contaminants that cause them. Expensive nutrient removal technology, for example, is irrelevant if the problem is microbiological contamination.

Treatment plants are the most commonly propounded measure to address environmental degradation from sewage. Such treatment can indeed be highly effective, but should not be seen as a universal solution.

In many situations, particularly in the developing world, there are simpler, less capital-intensive, and more financially and technically sustainable alternatives that may provide better environmental outcomes, both with respect to sewage pollution and by allowing investment to be diverted to address other environmental problems.

There is still a need, however, for continuing development of innovative and appropriate solutions.

Diffuse Sources

Sewage treatment is an option only if there is a reticulated sewerage system to collect the sewage and deliver it to the treatment facility.

In fact, this is more he exception than the rule. In many developing countries – and indeed some developed ones – only a minority of the population is served by reticulated sewerage systems, even in urban areas: the number of people without adequate sanitation is not expected to decrease before 2030 even with accelerated investment.

Constructing municipal sewerage requires substantial capital investment, which is often not available. Even when capital is available, it may not make economic sense to invest in treatment facilities prior to completion of the reticulation network (e.g., in the Philippines).

Rapid urbanization in many coastal areas, often in the form of unplanned squatter settlements, adds to the difficulty of providing sewerage and treatment infrastructure.

In such circumstances, providing water supply usually has a higher priority than sewage collection and treatment. Neighbourhoods are often provided with a municipal water supply before they receive sewerage to dispose of the increased volume of wastewater that results

As a result, non-point sources such as septic fields, and pit or overwater latrines, are a significant source of sewage contamination in many areas. In many countries significant reductions in sewage contamination could be achieved by converting pit or overwater latrines to septic tanks, by better design and construction of existing septic tanks, or by better provisions for septic sludge disposal.

The failure of on-site systems because of poor ongoing operation and maintenance (e.g., not emptying tanks or pits) is a common reason given for needing sewerage.

Septic tanks can also be linked to stepped digestion tanks that produce effluent suitable for irrigating home gardens. There are also simple technologies, such as composting toilets and biogas generation, that are suitable for application in individual households or to small groups of them.

Depending on circumstances, measures concerning such on-site systems can have significant advantages over centralized reticulation and treatment systems. They are less expensive than conventional sewerage systems, especially at relatively low population density and can be implemented in smaller increments and with shorter lead times.

They can also be implemented at the community or even individual level, while ongoing operation and maintenance are often less financially and technically demanding.

Furthermore, equipment can often be manufactured locally.

Even when reticulated sewerage and sewage treatment is the best long-term approach to sewage management, on-site systems may be useful interim measures, and may enhance the system in the long term. In "settled sewerage", for example, septic tanks are used to pre-treat wastewater before it is discharged to a central system, reducing the load on it.

On-site systems do have disadvantages, however. Soils have a finite capacity to absorb septic effluents. This varies widely with soil characteristics, and in some places soils are unsuitable for septic tanks. Septic tanks are also relatively poor at disinfection. They can lead to microbial contamination of ground water – a negative impact, especially where wells are an important source of drinking water. Sewage contamination of wells, for example, has been identified as the highest regional priority in Eastern Africa.

Another useful measure in managing the impacts of diffuse sources of sewage is to take advantage of the capacity of artificial or natural wetlands to assimilate and retain wastes and remove pathogens.

Again, however, [this capacity] is not unlimited; when it is exceeded, the wetland can be degraded. Such assimilative capacities are poorly known, particularly in the context of long-term variability.

For this reason, and because natural coastal wetlands are both ecologically very important and widely threatened, using existing wetlands for sewage treatment should be approached with considerable caution.

The construction of artificial wetlands, on the other hand, increases the extent of coastal wetland habitat, often generating cross-benefits, but requires considerable areas of land. The use of wetlands for sewage treatment may also be incompatible with other uses, such as food production and recreation.

Point Sources: Wastewater Outfalls, Animal Husbandry, and Industry

The costs and technical capacity required to construct, operate, and maintain sewage treatment systems increase with progressively higher levels of treatment: but contaminant removal efficiency does not, except for nutrients.

The increased cost of tertiary treatment is therefore justified only when nutrient input is a significant environmental concern and sewage is important relative to other nutrient sources.

Tertiary treatment is particularly likely to be required when a series of cities discharge effluents down the course of a river, producing a cumulative increase in nutrient levels.

Where, however, effluent is discharged into particularly sensitive areas such as tropical lagoons, even relatively advanced tertiary treatment may not reduce nitrogen concentrations to a level that removes the threat of eutrophication.

Where waters are used for bathing or producing seafood, protecting human health is often a primary objective. Disinfection can reduce the numbers of bacterial indicator organisms by more than 99%, depending upon the nature of the effluent. There are so many microbes in untreated sewage, however, that large numbers may remain even after very high percentage reductions.

Furthermore, standard indicator organisms such as coliform bacteria are not necessarily reliable indices of pathogen levels. It is therefore good practice to locate sewage outfalls well away from bathing beaches, shellfish beds, and similarly sensitive areas, even if the sewage is disinfected before discharge.

It is also important to consider the possible harmful effects of disinfection methods, such as chlorination, that can leave harmful residues.

[Deep ocean outfalls]

Placing effluent discharges appropriately is often effective in reducing the environmental impacts of a given level of treatment – or in reducing the cost of treatment necessary to achieve acceptably low impacts.

Deep ocean outfalls are a viable option for many, if not most, coastal cities. Offshore outfalls often distance the discharge from bathing and recreational waters and fishing grounds and, depending upon local water circulation, maximize dispersion and dilution. They require much less ongoing technical support and expense than advanced treatment plants, and have a lower frequency of failure.

This is a particularly important consideration for developing countries with low capacities to maintain treatment plant performance. [A 1993 study], for example, reported treatment facilities in Pacific Island nations that discharge effluent of no better quality than raw sewage.

Given that tertiary treatment may not adequately safeguard against eutrophication, even when plants are performing to specifications, plant failure can be expected to have severe negative effects. In such cases, an ocean outfall is likely to provide a better, more certain, and more cost-effective environmental out-come than the construction of a treatment plant. [...]

The performance of any particular sewage treatment system will depend upon a number of factors. One is the characteristics of the raw waste stream, and, in particular, whether or not it includes industrial waste. Domestic sewage treatment systems typically fail adequately to remove POPs, radionuclides, and some other trace contaminants, but the levels of these are usually low if there is no industrial component in the waste stream.

Some industrial wastes, such as some POPs, may actually interfere with domestic waste treatment, for example by poisoning biological digestion. When raw sewage is discharged with industrial waste, lipophilic chemicals in the latter adsorb to the organic matter: the two are subsequently transported together and inextricably linked. It is therefore usually preferable to treat industrial and domestic liquid waste streams separately.

In practice, the effective level of sewage treatment is usually determined on the basis of socioeconomic conditions rather than through objective analysis of environmental protection needs. As noted above, high costs prevent much of the world’s population from being serviced by any form of sewerage system. Sometimes there are also distortions in the allocation of investment in sewage treatment. Some small island developing states, for example, are denied access to concessionary financing for treatment facilities because of relatively high per capita incomes.

Conversely, there may be excessive investment in treatment infrastructure. A simple offshore outfall may be perceived locally as "second-class" technology. Engineering and construction firms aggressively market advanced treatment systems. International donors often have a predisposition for capital-intensive infrastructure projects and developed-world solutions, but have budgetary constraints that prevent long-term investment to build the capacity needed to sustain treatment plant performance. Some donors require that treatment plants be constructed for all development projects, usually with a level of technology appropriate for the donor – but not necessarily the recipient – country. These pressures should be resisted in favour of a realistic assessment of environmental risks and of the sustainability of treatment performance. Where large infrastructure projects are undertaken, there should be adequate provisions for capacity building and sustainable financing to support long-term performance.