Primary Treatment in Onsite Systems: Factors that Influence Performance

Methodology, preliminary results and future plans for an on-going Water Environment Research Foundation (WERF) funded research project, WERF project number 04-DEC-7, are presented. The objective of this project is to identify, compile, analyze, and report on the existing body of literature and other data sources addressing the performance of primary treatment units (septic tanks and grease traps) in onsite wastewater systems and the factors impacting performance.

To date, a bibliographic database has been developed and approximately 470 relevant references have been identified and/or collected. The database is being used to manage information for the project, including review input from a team of four leading practitioners and researchers in the field. The references, which include texts, guidance manuals, journal articles, conference proceedings, demonstration system reports and as-yet unpublished data, cover a wide range of factors influencing primary treatment performance in onsite systems. Factors which have been studied in the past and will be closely examined for this project include: sizing, compartmentalization, effluent filtration, baffling, hydraulic loading, water conservation, additives, nutrient removal, temperature effects, solids accumulation rate, removal of trace organics, settling theory and many others.

The results of this extensive literature/data review will be compiled in a comprehensive white paper supplemented by the bibliographic database. Additionally, an extension-style communications piece will be developed to communicate the results of the project to practitioners and other stakeholders in a practical way that facilitates their decision-making
regarding primary treatment units in onsite systems.

Approximately 23 percent of the estimated 115 million occupied homes in the United States are served by onsite wastewater systems (U.S. Census Bureau, 1999). The vast majority of onsite wastewater treatment systems include a septic tank, grease trap, or both for primary treatment. These units are efficient, simple, low/no energy treatment units whose performance is critically important to the overall functioning of onsite wastewater systems. State regulations and most sanitary and wastewater engineering texts contain design, construction, and installation criteria for these units, although such standards tend to vary widely and are often incomplete in their consideration of the myriad factors that may influence primary unit performance in onsite wastewater systems.

Primary treatment units, such as septic tanks and grease traps, may precede additional secondary or advanced external treatment units, such as biological filters, constructed wetlands, aerobic treatment units, and disinfection systems or they may directly precede discharge to a soil-based treatment and disposal system. Secondary or advanced pretreatment systems and especially soilbased treatment systems can be quite sensitive to the quality of the primary effluent they receive. The infiltrative surfaces of biological filters and soil-based treatment systems may be rapidly clogged by grease or bound by solids, necessitating extensive, drastic, and costly repairs, and potentially creating environmental contamination problems and public health risks. The performance of the primary units in an onsite wastewater system with regards to effluent quality is therefore absolutely critical, though frequently overlooked due to the relative simplicity of these units and the tendency to focus attention on more glamorous aspects of the onsite system design.

Primary treatment units serve other important functions in onsite wastewater systems. In addition to removing suspended solids, fats, oils, and greases (FOG), and associated organics and pathogens, these primary treatment units effect substantial reductions in solids volume via digestion and may transform dissolved constituents biologically or physiochemically. Furthermore, they provide for the dilution of pollutant spikes and the attenuation of peak flows, both of which are often critical for the optimal performance of downstream processes.

While primary treatment units in onsite systems are of relatively simple design and operation, they are of such fundamental importance to overall onsite system performance that a thorough understanding of the factors affecting their performance is needed. The myriad factors affecting primary treatment unit performance can be subdivided into categories, as listed in Table 1.

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