May 2008, Vol. 20, No.5

Small Communities

Minimizing Infiltration and Inflow in Alternative Systems

Jim Kreissl and Bill Bowne

Those of us who have lived through the U.S. Environmental Protection Agency construction grants program remember the old adage that “it is cheaper to accept infiltration and inflow (I/I) in the sewer and treat it than to remove it.” Today, this rule of thumb is no longer true, as conventional gravity sewer systems have become both overloaded and quite expensive to enlarge. The advent of alternative collection systems that use a variety of onsite processes and shallow and smaller plastic pipe collection systems has been heralded as an alternative to conventional collection systems that can greatly reduce both I/I and capital costs.

Those who read the Water Environment Federation 2008 Manual of Practice on alternative systems will note an emphasis on the quality assurance for proper inspection and construction of the on-lot portion of these systems. Quite simply, these systems are not designed to accommodate significant clear water volumes. High wet weather flows from excessive I/I also can overwhelm these alternative collection systems, as they do in conventional collection systems. The recognition of this potential dictates that the engineer should evaluate existing I/I potential and require its minimization during the construction phase of the project.

To understand the nature and solution to this problem, one needs to understand the sources of I/I in conventional systems and the ill-conceived regulatory codes that facilitate this problem. Most I/I studies of conventional sewers point to the three primary sources of extraneous flows: leaking manholes and lift stations; imperfect joints in the deeply placed sewer pipe; and inadequate oversight and construction practices, including illegal connections to existing building drains and building sewers. Paying attention to this last source of I/I is important in ensuring the successful performance of alternative collection systems.

Although one can assume that new housing will have acceptable plumbing, building drains, and (usually) building sewers, these assumptions cannot be made when alternative systems are applied to existing older housing units. However, the cost of assessing these on-lot sources and their removal still is generally small when compared to the capital savings that accrue from choosing an alternative system in nonurban areas. Since funding issues are always a big concern, engineers and system owners sometimes have failed to expend the needed effort and money to ensure that I/I sources from illegal connections and old crushed pipes are minimized. Generally, they do not understand the importance of the issue and the additional design and construction practices that can be incorporated to minimize potential problems.

The standard design rules for conventional sewers call for manholes to be placed every few-hundred feet along the length of sewer and pipe to be laid at sufficient grade to maintain a minimum velocity of flow. The latter requirement effectively drives the sewer deeper into the ground, where construction cost is proportionally higher and the chances of intercepting groundwater increase. Many of these same rules require a test to show that the infiltration rate into such a sewer in dry weather is equal to or less than 450 L (120 gal) per capita per day. If it meets that requirement, the system is considered acceptable. The problem is that recent studies show the average water use is less than 265 L (70 gal) per capita per day, and the “acceptability test” allows 190 L (50 gal) per capita per day, or more than 40% of the flow, to be from clear water sources during dry conditions.

An even worse example requirement is that such sewers are allowed to carry 1040 L (275 gal) per capita per day during wet weather (storms or melting), or essentially a 300% increase above the residential wastewater load. This figure is supposed to include the inflow contribution from illegal on-lot connections (such as footer and roof drains and sump pumps), leaking manholes and lift stations, and additional rainfall-induced infiltration through the same belowground openings as normal infiltration.

Since these are finished-construction-project acceptability criteria, one can only imagine what these values were before the I/I reduction project was initiated. The relative values for source contributions to the entire system vary based on climate, materials and methods of construction, system age, soil conditions, and hydrology.

So, how does this brief review apply to alternative collection systems, such as vacuum sewers, grinder-pump pressure sewers, and septic tank effluent pumping (STEP) and gravity systems? All three generally employ much longer lengths of gasket-joined polyvinyl chloride or butt-welded high-density polyethylene pipe buried at a shallow depth (usually about 760 mm [30 in.]), and fewer shallow cleanouts (with much lower capital costs) are employed instead of multiple manholes and lift stations. Elimination of the latter two conventional sewer components significantly reduces infiltration and, to a lesser extent, inflow. With fewer and better pipe joints normally located above the water table, infiltration is possible only if the water table rises during a wet period and if pipes either are located below standing-water locations or have been located to serve as a sort of French drain that unintentionally intercepts surface-water runoff from surrounding tighter soils.

The nature of all alternative systems is that some form of treatment or motive force device is located on the served properties. Significantly reduced capital costs of these systems, as compared to conventional systems, have been the primary reason for their adoption by hundreds of communities and private and public utilities. However, the desire for the lowest possible capital costs sometimes has resulted in operational problems in systems designed to serve existing neighborhoods. In almost every case study of alternative system adoption in existing communities formerly served by septic systems, the issue of whether to use existing on-lot components is discussed. Even when this option is discarded, many communities stop service lateral installation at the inlet flange of the first on-lot device for each system. This leaves the building drain and building sewer, where most of the clear water sources are located out of the construction plan. Such decisions are an invitation to system performance problems from excessive clear water inflow.

The only known work to quantify this problem has been performed by Bill Bowne on three alternative collection systems located, respectively, in Dexter, Glide, and LaPine, Ore. In the Dexter system, the decision to use existing building sewers and inadequately designed and imperfectly installed septic tanks has resulted in peak wet weather flows of seven times the average dry weather flow in this system. System corrections are under way by repairing these components. In Glide, each septic tank was new, of high-quality construction, and tested. The ratio of peak wet weather flow to average dry weather flow was only 2:1, and that small increase was attributed largely to some riser problems due to settling. The LaPine STEP system seems to have a very acceptable level of I/I (<2:1) due to unique pumice soils, and all of the on-lot facilities were carefully overseen to ensure system performance. These systems still have some difficulties with perched groundwater or partially thawed soil conditions that temporarily cause an effective groundwater table to extend to the ground surface, but only the Dexter system has been unable to reduce wet weather flows to desirable levels.

Alternative collection systems have several advantages over conventional ones, but engineers must properly evaluate building drains and building sewers for I/I before finalizing their project design to get the full value of their use. These efforts should yield an essentially watertight system that will provide a cost-effective wastewater collection system for many years without having to collect and treat clear water, as conventional systems must do. Thus, a properly designed and constructed alternative collection system can offer a significant reduction in collection system I/I, which is the leading problem for conventional system treatment operators.

Jim Kreissl is an environmental consultant in Villa Hills, Ky. Bill Bowne is a registered engineer in Eugene, Ore. The authors would like to thank Rich Naret of AIRVAC (Rochester, Ind.) and Richard Otis, a consulting engineer in Madison, Wis., for their review comments on this column.

Jim Kreissl is an environmental consultant in Villa Hills, Ky. Bill Bowne is a registered engineer in Eugene, Ore. The authors would like to thank Rich Naret of AIRVAC (Rochester, Ind.) and Richard Otis, a consulting engineer in Madison, Wis., for their review comments on this column.