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Will the drainfield work? Dr. Davis describes just how computer modeling can be used as a predictive tool for engineered drainfields. This effort can save time and money when correctly applied to certain hydrogeological conditions.

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The decentralized wastewater market has arrived in a big way in North America, with new developments and small existing communities embracing decentralized technologies. The growth of this market has been aided by intensive research and development. Numerous decentralized treatment technologies have been introduced to the market in recent years, including engineered wetlands, package aerobic units, membrane bioreactors and textile-based trickling filters. Although the recent decrease in housing starts in the United States may have tempered the growth of the decentralized market for the short term, the predicted increase in population from the present 301 million to 419 million persons by 2050 bodes well for the long-term prognosis of the decentralized wastewater treatment industry.

Designing a subsurface distribution system

One of the primary concerns of designers, regulators and operators is the potential for the treated wastewater distribution system to fail, defined by the ponding of treated wastewater at the land surface due to low permeability soils or the surfacing of the water table.

Proper design of a subsurface distribution system involves a number of critical steps. The design flow must be known, the acceptable hydraulic (linear) water loading rate to the subsurface must be determined, the distribution pipes must be correctly sized in terms of length, diameter, orifice diameter, and orifice spacing, and pumps must be sized at the appropriate flow rate. The linear loading rate is determined through a soils analysis obtained from soil borings and test pits, with the soil being analyzed for redoximorphic features, color, texture, structure and consistence. In Minnesota, these results are used to determine the soil type (e.g., sand, silt loam), with each type correlated to a linear loading rate (gallons per sq. foot per day). The...

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