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.
