THE DECENTRALIZED CONCEPT OF
by David Venhuizen, P.E.
This quote from the World Bank publication Sanitation and Disease challenges the "conventional wisdom" that centralized wastewater systems are an inherently 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".
Definition of the Concept
The decentralized concept of wastewater management provides a framework for producing "alternative" systems which meet this need-systems which in many situations may also be 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-and beneficially reused, where this is practical-as close to where it is generated as practical to eliminate a costly pipe network that does nothing but move pollution from one place to another.
The most visible current example of the decentralized concept is the familiar septic tank/soil absorption system installed on individual lots. In fact, many in the wastewater field see management as a dichotomy in which the ONLY choices are individual on-site systems under the sole control of property owners, or a centralized sewer system. Therefore, when people think of decentralized management, they most often presume the discussion is about individual septic systems, and it is just this one extreme of that "dichotomy view" which the suggestion of decentralized management conjures up in their minds. On-site systems-especially "alternative" systems (like sand filter treatment and drip irrigation dispersal) which can cope with severe site limitations and effectively provide beneficial reuse-may indeed be components of an areawide decentralized concept system. However, the decentralized concept goes beyond merely managing individual on-site systems. The decentralized concept adopts a "continuum view" that expands the choices, filling the gap between on-lot systems and conventional, centralized systems.
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 prior to dispersal or discharge, and those processes may be deployed at various levels of centralization. Final treatment and/or dispersal components might be installed for each house, for each block of houses, for a whole subdivision, or for an entire community, and the arrangement could vary from neighborhood to neighborhood within the jurisdiction of a management authority. Many considerations would determine how close to the source of generation it is practical to address treatment and reuse, dispersal or discharge. 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". Other considerations include topography, soil conditions, development density, type of land use and community desires in regard to land use, and points of potential reuse and/or points where discharge would be allowable or beneficial.
It is important to understand however that the ENTIRE system, no matter how the components are 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 concept system is no different in this regard.
Technologies for the Decentralized Concept
Decentralization dictates that the overall system would be composed of many small treatment centers. To minimize the operations and maintenance liabilities imposed by this strategy, judicious choice of technologies is required. The conventional mindset typically thinks only of activated sludge package plants for such applications, but experience has shown that proliferating many small package plants would produce untenable operations and maintenance liabilities. The liabilities of that strategy were, in fact, a driving force for the "regionalization" policies in many states. To minimize these liabilities, treatment technology must be appropriate to the volume of flow, the nature of the development served, the nature of the reuse opportunities, and limitations on dispersal or discharge options.
Most "mainstream" practitioners are unfamiliar with the most appropriate technologies, so there is typically not the same level of comfort with these technologies as there is with mainstream technologies used in conventional, centralized systems. With a better understanding of these alternative technologies, however, engineers and system managers may come to appreciate their value.
For example, from a "conventional" point of view, installing a septic tank at each wastewater source might seem like a questionable course of action. But this strategy offers several benefits. Septic tanks retain settlable solids, allowing the use of smaller, less costly effluent collection systems. Septic tanks operate passively, requiring only periodic inspection and pumping. Because digestion in septic tanks is anaerobic, the volume of solids that must be handled would be reduced significantly compared to conventional aerobic processes used in centralized systems. Properly sized tanks can hold solids for several years before any handing is necessary.
With the sources of sludge being dispersed, the logistical challenge of sludge management is certainly not to be minimized. However, the timing of sludge removal is not critical. Usually, many months could pass between the time when monitoring indicates that a septic tank should be pumped and the time it is actually pumped without significantly affecting downstream components of the system. This is in sharp contrast to the most often used conventional treatment processes, in which sludge handling must be addressed on a time scale measured in hours or the process will degrade and begin releasing poorly treated effluent.
Further, this isolation of sludge sources may enable more cost-efficient disposal or beneficial use of the biosolids. The use of sludge from centralized plants for soil improvement often is resisted because toxic substances might be concentrated in it. When sludge is collected from dispersed septic tanks, it can be classified by source, and material that may contain toxic substances can be isolated from the rest of the sludge, potentially enhancing beneficial use of sludge from "safe" sources.
A final advantage is that the timing of removal is not critical. Tank pumping could be scheduled when the biosolids could be readily used. This would eliminate the need for intermediate storage and handling facilities.
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 and 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, a technology that can upgrade septic tank effluent to advanced secondary or even tertiary quality. Many trophic levels of organisms live in and on the filter bed, and the fine-grained media provides a very large surface area relative to the rate of flow through the system, making the process relatively "low-rate". Process failure-clogging of the filter bed-occurs slowly and gradually because of these characteristics. Eventually, wastewater will no longer flow through the bed at the rate it is being dosed onto the bed. Since this condition builds up slowly, an operator would have ample time to plan and execute maintenance procedures before clogging reached a point where treatment degraded or bypasses might occur. When properly designed and loaded, maintenance requirements would be minimal and a filter can operate for several years before maintenance is required. 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.
In contrast, mechanical-biological processes like activated sludge, which are typically employed in centralized treatment plants, depend on few trophic levels of organisms living in concentrations far higher than found anywhere in nature, rendering the process inherently unstable. Constant inputs of energy and close attention to operating conditions are required to keep the process "on track" and even the most conscientious operation is often not able to do so. Any problem-an accident, equipment failure, large flow variation, untimely maintenance-typically leads to a process breakdown and, because the process provides no physical barrier to its passage, subsequent release of poorly treated effluent.
In addition to sand filters, several other technologies possess "fail-safe" qualities. These include constructed wetland concepts 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 concept offers several environmental, fiscal and societal benefits relative to conventional practice.
Centralized systems concentrate large flows through one pipe, lift station or treatment plant, so mishaps tend to have "large" consequences. But in a decentralized system, flows at any point would remain small, implying less environmental damage from a mishap. In any case, bypasses, leaks and overflows would be far less likely in a decentralized system. Treatment technologies would be more "fail-safe" and lift stations would be minimized. Carrying only liquid effluent to dispersed treatment centers, the collection system would consist of shorter runs of smaller pipes containing fewer openings, providing less opportunity for infiltration, exfiltration and overflows.
Installation of a decentralized system would cause less environmental disturbance. Smaller collection system pipes would be installed at shallow depths and could be more flexibly routed. Large interceptor mains, which typically run in creek bottoms, would not be needed, so riparian environments would not be disturbed.
Over the long term, environmental disturbance would be minimized because existing lines would not need to be reworked to upgrade system capacity. Expansion would be accomplished by adding new treatment centers rather than by routing ever more flow to existing centers.
Treatment and reuse can be tailored to the waste stream. Industrial waste, for example, can be treated separately rather than being commingled with domestic wastes. These generators can be required to implement treatment methods specific to their wastewaster characteristics and beneficial reuse opportunities.
The major fiscal advantage of a decentralized system comes from eliminating a great deal of the collection system infrastructure. Typically, most of the money budgeted for a conventional, centralized system is spent merely moving wastewater from one point to another. In a decentralized system, no large interceptor mains and few, if any, lift stations are needed. The collection system that remains, employing small-diameter effluent sewers, would be less costly to install than conventional collector mains, usually more than making up for the cost of septic tanks that enable the use of effluent sewers.
Because infrastructure costs would be reduced, resources can be redirected to appropriate treatment and beneficial reuse opportunities. Reuse of reclaimed water can deliver additional fiscal benefits, both to the individual user and to the community, by displacing potable water demands.
Decentralized concept systems would also incur lower maintenance costs. Little or no infiltration and inflow would enter effluent sewer systems, decreasing collection system maintenance costs and peaking loads on plants, perhaps allowing some components to be downsized. Since these sewers do not carry solids, sewer maintenance would also be essentially eliminated. And as noted previously, the technologies favored for dispersed treatment centers would typically require minimal maintenance.
Benefits to Society
Societal advantages also may ensue from the use of a decentralized concept system. Beneficial reuse of reclaimed water would become more cost-efficient because effluent would be available throughout the service area, nearer to potential points of use, decreasing the cost of a reclaimed water distribution system. Such non-potable demands as landscape irrigation, toilet flush supply and cooling tower makeup could be served with reclaimed water.
A decentralized concept system could accommodate any level of water conservation found to be economically attractive or ecologically necessary. Because only liquid effluent is transported, reduced wastewater flows resulting from water conservation measures would not cause clogging problems in the collection system, as has occurred in conventional, centralized systems.
The decentralized concept would be easier to plan and finance. Each project would be small compared to the typical regional system expansion. Management needs of each area or new development would be considered directly and could be implemented independently. Much of the cost could be privatized or assigned directly to development generating new demands rather than being borne by the entire community through bonds. Capacity expansion-and therefore capital requirements-would follow demand much more closely, minimizing funds expended for facilities that may not be fully utilized for years to come, as occurs routinely in conventional, centralized systems.
Decentralized concept systems can be designed and installed in a growth-neutral manner, responding to development only as it occurs, whereas centralized systems often spur growth, even requiring it to be fiscally viable in many cases. As noted previously, the decentralized concept readily accommodates the use of different strategies in various parts of the service area. This enables system planners to respond in the most fiscally efficient and environmentally responsible manner for each set of circumstances.
Decentralized systems are not the answer to all wastewater problems. Just as the "one-size-fits-all" mentality of conventional, centralized systems is unreasonable, so too will there be cases where the decentralized concept would not make the best sense. Still, the potential benefits of this management approach indicate that it deserves greater attention, especially in smaller communities and in the developing urban fringe.
Barriers to broader implementation of the decentralized concept appear to be mostly institutional, not technical. These barriers command the attention of policymakers, regulators, operating authorities, engineers, land developers and the general public. Given the water resources challenges of today, it is time to rationally analyze all possible management strategies, not merely those accepted as "conventional wisdom".