Municipalities and Developers Eye Better Land Use Through Drip Distribution

About the author: David Linahan, P.E., is a chief sanitary engineer for Yerkes Associates, West Chester, Pa.

David Linahan, P.E.

While the face of Pennsylvania’s rural communities continues to change from bucolic farms to mushrooming suburbs, new residents are asking their municipal leaders to preserve as much of that original pastoral setting as possible. While that can prove to be a constant battle between preserving green space and building new roads, schools and shopping centers, one area that holds promise for resolving those competing demands is in the area of municipal sewage treatment.

Since 1972, with the passage of P.L. 92-500, the choice for effluent disposal from municipal sewage plants has been moving from stream discharge to some form of land application. Although spray irrigation increasingly has been the method of choice in many regions, it has some inherent disadvantages when compared to modern, improved methods of subsurface drip distribution. Subsurface distribution methods of land application have been preferred mainly because there is little chance for human contact with the effluent after dispersal.

A drip distribution system consists of dripper tubing installed in parallel lines 6 to 10 inches below the ground surface and along the contour. Sophisticated emitters, located every 6 to 24 inches inside the tubing, uniformly apply treated wastewater, or effluent, into the most biologically active horizon of the soil. It does this while keeping the soil unsaturated. The placement of the dripper tubing in the upper soil horizon promotes ideal conditions for further polishing the effluent by the natural biological reduction of organic and nitrogenous compounds, while at the same time substantially reducing fecal coliforms and facilitating phosphorous fixes to the soil.

On the other hand, a spray irrigation system applies effluent to the ground surface with a system of spray nozzles. The nozzles often are three feet above the ground and spread out across a field to uniformly distribute the effluent.

Typically, the wastewater is applied in large doses to completely saturate the soil and ground cover, almost to the point of runoff. This is followed by a rest period of about one week to allow the field to dry out before another application. The alternating soaking and resting periods provide an environment that allows the ground cover plants to polish the effluent. Wastewater should not be applied during periods of high wind as off-site misting will occur. In addition, during and after a rain event, wastewater should not be applied until the rain stops and the field has a chance to dry out.

Chief among the disadvantages of spray irrigation is the need for large storage lagoons. Other disadvantages of spray include the large setbacks between the edge of the spray field and adjacent properties and restrictions on the active and recreational uses allowed within the spray field itself.

Spraying is limited in the rainy months during the spring and fall and often prohibited in the winter months due to below freezing air temperatures. Continued application of effluent into the freezing air will cause a build up of frozen effluent on the ground, like a snow-maker. This “snow” eventually will melt and cause uncontrolled runoff and soil erosion. Therefore, a municipality has to build and maintain a holding lagoon large enough to store 90 to 120 days of effluent until it can be applied when favorable conditions return.

Drip distribution occurs below the ground surface and, as long as the soil remains unsaturated, effluent can be applied throughout the winter. Ground cover and even snow will act as an insulator above the tubing. Additionally, the effluent applied to the treatment system typically is well above average soil temperatures even in the summer months. Any residual moisture that does freeze near the ground surface will not interfere with the applied wastewater’s ability to permeate through “frozen soil.” Since effluent does not have to be stored, lagoons for drip distribution can be much smaller or eliminated when compared to corresponding lagoons for spray irrigation.

The lagoons also attract large flocks of geese that have become a real problem. These lagoons become overrun with waterfowl that are no longer migrating south but are staying year round.

Advantages

The effluent application rates for drip distribution are nearly double those for spray irrigation systems. This is because in the drip method the effluent is applied in small micro-doses that more closely match the soil’s natural permeability. With spray,the effluent is applied in very large macro-doses to the point of soil saturation and almost to the point of runoff. Drip distribution also eliminates the problem of offensive odors, aerosol drift outside the distribution area and large setbacks.

Developers should be attracted to the advantages of drip distribution for some of the same reasons as municipalities. Since drip distribution requires less land for effluent storage and the dispersal area due to higher application rates and smaller setback requirements, developers can devote more land to lot development and help offset the cost of providing high quality treatment systems. Since runoff is not an issue with drip fields, they can be constructed on steeper slopes. Steeper slopes generally are not suitable for lot development. Therefore, a larger percentage of suitable land for homebuilding is preserved when using drip distribution.

However, I must admit that municipalities and state regulatory agencies have raised legitimate concerns about drip methods in the past. I believe all those concerns have been addressed with recent technological advances and a better understanding of how drip distribution really works.

When comparing drip to other subsurface distribution systems, improved filtration methods now can reduce the possibility of solids escaping and plugging the piping distribution system in the field. (See Figures 1 and 2.) This eliminates the need for back-up or reserve areas.

Advanced dripper tubing with pressure compensating emitters maintains a constant flow of effluent regardless of changes in hydraulic pressure in the system or the elevation at the point of application. Therefore, saturation is prevented and a uniform distribution from the top to the bottom of the hill is provided.

With uniform distribution unabated by seasonal variations or climatic changes, drip distribution is very easy to control. You just set it and forget it. The plant operator does not have to keep monitoring and worrying about when to spray and how much to spray.

Perhaps the biggest obstacle to adopting drip distribution methods over spray irrigation is simply force of habit. Drip only has been around for 25 years, compared to spray irrigation’s first reported use in 1881.

In 1965, the Israelis developed drip distribution out of sheer necessity to conserve their precious water supply. In Israel they recycle almost all their sewage back to the farms and apply it mostly through dripper tubing. Although Pennsylvania is not a desert, water conservation still should be a noble goal to achieve. When using spray irrigation, a lot of water can be lost through evaporation and uncontrolled runoff. With drip, all the water goes into the ground.

Regulators not yet convinced that drip will work when the ambient air temperature drops below freezing currently are requiring a minimal amount of storage. Our observations with the large volume drip system in Thornbury Township, Chester County, Pa., and studies conducted by the University of Wisconsin–Madison have shown that drip has worked throughout the winter. The ground around the dripper line does not freeze, or at least does not impede the dispersal of effluent.

“With proper design and installation, drip distribution systems are an excellent alternative system for wastewater dispersal in cold climates,” author Rachel Bohrer said.1 However, as a preliminary step, there are very inexpensive in-line heating systems that can be installed to guarantee a flow of warmer water that will penetrate any weather-related soil condition. In addition, letting grass over the field grow a foot or two longer in the fall will create a natural insulator to further prevent the soil from freezing.

Issues

Other issues regarding drip distributions that I now believe have been addressed are disinfection and nutrient removal, namely nitrate-nitrogen. When spraying, the effluent first must be disinfected, typically with chlorine, before being dispersed. Since dispersal takes place in the open air, there is a greater potential for human contact with the treated effluent. Disinfection is not necessary with the drip process. Therefore, there is a labor and chemical cost savings. In fact, the routine addition of disinfectants such as chlorine will impede the natural biological activity in the soil.

While nitrogen removal (denitrification) is a natural function mainly performed by plant life growing in the dispersal field, that function diminishes during the winter months when plant growth slows.

Much of what municipal engineers know of nitrogen removal by plant growth comes from agricultural science. Because it often is not practical to attempt to grow crops in the winter, there is little documentation on nitrogen uptake by plants during the winter.

In the turf industry there is a great need to develop grasses and techniques that will extend the growing season and keep grass growing all year round. Turf grasses need a lot of nitrogen and other nutrients common in wastewater. As turf scientists continue their research to grow grass in the winter, we will benefit from knowing how much nitrogen these plants can use during their extended growing season. In the mean time, treatment systems using denitrification processes are a safe and reliable alternative. 

I believe both spray and drip distribution will continue to co-exist, but where municipalities feel particular pressure on land use, drip can be viewed as an effective and efficient land-use alternative.