The Decentralized Concept Of Wastewater Management

David Venhuizen, P.E.,
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Austin, Texas 78745 USA
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"Those whose job is to select and design appropriate systems for the collection and treatment of sewage ... must bear in mind that European and North American practices do not represent the zenith of scientific achievement, nor are they the product of a logical and rational process. Rather, [they] are the product of history, a history that started about 100 years ago when little was known about the fundamental physics and chemistry of the subject and when practically no applicable microbiology had been discovered.... These practices are not especially clever, nor logical, nor completely effective--and it is not necessarily what would be done today if these same countries had the chance to start again."

This quote from the World Bank publication Sanitation and Disease challenges the "conventional wisdom" that centralized wastewater systems are the obviously superior method of management, highlighting that they really just derive from over 100 years of sanitary engineering tradition. In the beginning, due to a growing awareness of disease risks caused by the squalor in large cities, the focus was on piping wastes away from the population. Only later, as it was seen how the discharge of raw wastes had transformed rivers into foul open sewers, was treatment at the end of the pipe integrated into the management system. The result of this tradition is a "pipe it away and dump it" syndrome. This is currently the paradigm for wastewater management which is generally accepted by all levels of the institutional infrastructure that address this function--engineers, operators, regulators, administrators, etc.

Though the criticism quoted above is offered in the context of disease control, it can be argued that these thoughts apply equally well to many fiscal, societal and environmental aspects of wastewater management. Over the years, many have argued that a "sewerless society" would be the means of minimizing the problems, but most of these arguments proposed the use of composting toilets and other non-standard plumbing, methods which would enforce lifestyle changes that most people who grew up with "modern plumbing" would not readily accept. To be broadly applicable, decentralized management methods must be as "transparent" as practical to the user, allowing them to -- for a fee -- flush it and forget it, just as they are able to do when served by "the sewer".

The decentralized concept of wastewater management aims to provide a framework for producing "alternative" systems which meet this need--systems which are also more fiscally reasonable, more socially responsible, and more environmentally benign than conventional practice. Stated most simply, the decentralized concept holds that wastewater should be treated as close to where it is generated as practical. The most visible current example of this concept is the familiar septic tank/soil absorption system installed on individual lots, and it is solely this extreme which the suggestion of decentralized management conjures up in the minds of many. While on-site systems--especially "alternative" systems which can cope with site limitations--may indeed be components of an areawide decentralized concept system, they are not acceptable solutions in many situations.

The decentralized concept goes beyond merely managing individual user systems, filling in the "gap" between on-lot systems and the conventional, centralized system. In general, as explained below, it is suggested that septic tanks indeed be used to intercept the flow from each generator at the wastewater source, but effluent from these tanks may be routed to further treatment processes, and these may be deployed at various levels of flow aggregation. Many considerations would determine how close to the source of generation it is practical to address treatment and disposal. One very important factor is if and how the wastewater could be reused in a beneficial manner, challenging the very concepts of "waste" water and "disposal" (For further discussion of this aspect of decentralized management, see Is "Waste" Water Reclamation and Reuse In Your Future?.). Other considerations include topography, soil conditions, development density (existing or desired), and type of land use.

It is important to understand however that the entire system, no matter how the hardware is arranged, should be managed by an authority with powers appropriate to the needs of the technologies employed. Conventional sewers, lift stations and treatment plants would not continue to perform their intended functions for very long if operations and maintenance were left solely to the discretion of the individual users. A decentralized hardware system is no different in this regard.

Decentralizing the hardware system implies there may be many small treatment centers. To minimize the operations and maintenance liabilities imposed by this strategy requires judicious choice of technologies which are appropriate to the volume of flow, the nature of the development served, the nature of the reuse opportunities, limitations on disposal options, etc. This is perhaps the hardest point for those invested in the prevailing paradigm to accept, as there is mistrust -- rooted in unfamiliarity -- of all but the "mainstream" technologies which are typically employed in conventional, centralized systems.

As noted, it is most expeditious to use as the first stage of the treatment system a septic tank located very close to the source of wastewater generation. This offers several advantages:

The remainder of the treatment components in a decentralized system should likewise incur minimal operations and maintenance liabilities. They should also possess characteristics which render them as "fail-safe" as practical, such as inherent stability due to biological diversity or a physical configuration which dictates that mishaps or temporarily poor operating conditions would not routinely lead to bypasses of poorly treated water.

An example of a more "fail-safe" process is the intermittent sand filter, an excellent technology for upgrading septic tank effluent to a very high quality. Many trophic levels of organisms live in and on the filter bed, and the very large surface area of the media results in the process being relatively "low-rate". These characteristics dictate that failure would be slow and gradual, with the mode of failure being clogging of the filter bed. Inherent in this concept of failure is that, rather than bypassing poorly treated effluent, water will no longer flow through the bed at the rate it is being dosed onto the bed. When properly designed and loaded, maintenance requirements should be minimal and filter runs of several years should be expected. Insightful design of the filter bed system allows the bed to be restored to normal function quite expeditiously at the end of a filter run, taking the bed out of service for only a few hours.

All this is in high contrast with the mechanical-biological processes typically employed in centralized treatment plants. Those treatment processes depend upon few trophic levels of organisms living in concentrations far higher than found anywhere in nature, rendering the process inherently unstable. Any problem--be it some sort of accident, equipment failure, large flow variation, or untimely operations and maintenance procedure--typically leads to a breakdown in the process and, because the process provides no physical barrier to its passage, subsequent release of poorly treated effluent in very short order.

In addition to sand filters, several other technologies possess "fail-safe" qualities. These include plant-rock filters, pond-wetland systems, and perhaps simplified and scaled down versions of the trickling filter. Those processes can be well suited to decentralized deployment by dint of their largely passive operation, diverse ecology, and--with proper design and safeguards--their ability to be flexibly sited.

Given the use of appropriate technologies, the decentralized management concept offers several environmental, fiscal and societal benefits relative to conventional practice. The following factors indicate that decentralized systems would be more environmentally benign:

A decentralized concept system would be less costly than conventional practice for the following reasons:

Societal advantages may also accrue from the use of decentralized concept systems. These include:

In closing, it should be noted that the decentralized concept will not be the answer to all wastewater management problems. Just as the "one size fits all" mentality of those who view conventional, centralized systems as the only reasonable approach is inappropriate, so too will there be cases where decentralized management does not make the best sense. Still, the many potential benefits of this alternative strategy indicate that it is a method which deserves much greater attention, especially in smaller communities and the developing urban fringe.

It is clear that most of the barriers to broader implementation of the decentralized concept are institutional rather than technical. These matters command the attention of policy-makers, regulators, operating authorities, engineers, developers and interested members of the general public. Given the water resource challenges encountered in many parts of the world, it is time to engage in a rational analysis of all possible management strategies, not merely those accepted as "conventional wisdom".

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