April 2008, Vol. 20, No.4

Small Communities

Distributed Wastewater Management: the More Sustainable Strategy

David Venhuizen

 

What do we mean by sustainability? Clearly, nothing is sustainable in an absolute sense. Even life on Earth, which will end in about 5 billion years, according to the best science now available, when the sun, expanding in its death throes, leaves Earth as a lifeless cinder. Much closer at hand, the specter of “peak oil” implies that many practices of our oil-fueled society cannot be sustained. So it is clear that nothing is absolutely sustainable — entropy never sleeps. When we talk about sustainability in regard to water resources management, then, we are speaking in relative terms, about how to lighten the footfalls of civilization on the ecosystem and how to lighten the burden inherited by future generations to maintain or alter the water resources management system. And it is here that distributed wastewater management comes to the fore, offering enhanced fiscal, societal, and environmental sustainability.

The environmental aspect is often the major focus. In this regard, sustainability — in the relative sense under consideration here — means that the methods and strategies would create a lesser negative impact on the ecosystem. Distributed wastewater management accomplishes this by minimizing disruption of the environment, decreasing vulnerability, and dispersing risk.

Risk dispersal is inherent in the concept of using many distributed treatment plants, rather than one centralized treatment center. A centralized plant presents a “large” risk due to any problem or mishap, while a problem at one much smaller distributed plant is a “small” risk. Similarly, aggregating large flows through large trunk mains and lift stations presents a large risk due to a break or failure, while in a distributed system, maximum flow at any one point would be much lower, thus entailing a small risk. Then, too, instead of there being large point sources of “permitted” pollution, a distributed treatment system usually distributes the treated effluent, placing small amounts in many places, thus minimizing impacts on the environment due to any pollutants remaining in the effluent. This is so whether the effluent is reused for an explicit purpose — such as irrigation — or it is simply dispersed as a disposal operation. Further, this distributed dispersal system often results in helping maintain the hydrologic integrity of a watershed, in itself a prime measure of environmental sustainability.

One of the defining characteristics of distributed wastewater management is that, by dispersing treatment and reuse, a lot of infrastructure that really does nothing but move pollution from place to place is eliminated. Typically, distributed systems minimize the number of lift stations and eliminate large trunk mains, which often run in riparian corridors. The collection infrastructure that remains is composed of smaller pipes running at shallower depths, particularly if effluent sewerage is employed, which also eliminates manholes. Further, a distributed system is expanded by adding more treatment centers, rather than by routing ever increasing flows to the centralized plant, so upgrading lines to increase capacity is never required. The nature of the system, then, results in a small amount of disruption to install, repair, or upgrade the system. It also imparts a small vulnerability to line leaks, line breaks, lift station failures, and manhole overflows.

Another aspect of vulnerability is the nature of the treatment processes. With there being many distributed treatment centers to oversee, distributed systems strongly urge the use of treatment processes that are more robust and more inherently stable, thus requiring less intensive oversight than the technologies typically employed at centralized treatment plants. These technologies are more likely to reliably produce an effluent of consistent quality with a minimum of operator intervention. These more robust technologies also typically require less energy to operate, and this reduces their carbon footprint, another aspect of environmental sustainability.

Many of the characteristics that impart environmental sustainability also make distributed wastewater management more fiscally sustainable. In general, the greatest capital savings would be gained by eliminating a large portion of the collection system since, in the typical centralized system, collection infrastructure accounts for about 80% of the total system’s capital cost. If effluent sewerage is used for the collection system that remains, this would result in further cost reductions versus conventional mains, lift stations, and manholes. Further, little or no infiltration–inflow should enter an effluent sewer, decreasing maintenance costs and peaking loads on treatment and dispersal facilities. And again, choosing robust treatment technologies can reduce the cost of operations and maintenance.

Besides these direct fiscal savings, further life-cycle savings can be realized by employing a “just in time” strategy of capacity development, which the distributed management concept makes possible. Rather than building infrastructure capacity for a long-term planning horizon, as is typically done when installing or expanding collection lines, lift stations, and treatment plants in a centralized system, the distributed management concept enables resources to be focused on serving only imminent demands, since the infrastructure would be serving only a relatively small targeted area.

Another fiscal consideration is the value of the water. In many regions, water supplies are being increasingly strained, a situation which planned reuse of reclaimed wastewater can help to address. Under the distributed management concept, effluent is produced at many points throughout the overall service area, potentially closer to points of reuse. In many cases, this can render reuse more cost-efficient by minimizing the cost of redistribution infrastructure to substitute reclaimed water for potable water. This practice also can reduce water treatment costs and water pumping and storage costs, and can forestall expansions of water treatment and storage facilities.

The distributed management concept also purveys societal advantages, each of which in its own way can help society operate in a more sustainable manner. Expanding reuse as just noted can stretch available water supplies. Each individual distributed system is a relatively small project which can be planned and implemented on much shorter lead times than can expansions of regional systems. The management needs of each area can be considered independently, and the costs of systems for a particular area can be more readily assigned to the activity generating the demand. Since each area is addressed independently, the overall centralized management program need not use a “one size fits all” strategy; rather, different approaches could be used in each, as best suits the characteristics of development and growth potential. This underscores that the distributed management concept is growth-neutral, whereas centralized systems often spur growth without regard to its desirability; indeed, they often require growth to be fiscally viable. Finally, if effluent sewerage is employed for collection, the system can accommodate any level of water conservation found to be economically attractive or ecologically necessary, since lines carrying only interceptor tank effluent would not clog if the flow were drastically reduced, as has occurred in conventional lines when drought required stringent water conservation.

The merit of employing distributed wastewater management strategies so as to attain these sustainability benefits will be dictated by the circumstances in each case, and by the boldness and creativity of the planners and designers. But it is clear that those sustainability benefits do urge all concerned to consider the distributed wastewater management concept in many — if not most — situations. Doing so can lead to wastewater management systems that are more fiscally reasonable, more socially responsible, and more environmentally benign.

David Venhuizen is an independent consultant in Austin, Texas, and a member of the Water Environment Federation (WEF; Alexandria, Va.) Small Communities Committee.

David Venhuizen is an independent consultant in Austin, Texas, and a member of the Water Environment Federation (WEF; Alexandria, Va.) Small Communities Committee.