January 30, 2006 | General

Using Compost To Treat Wastewater Effluent

BioCycle January 2006, Vol. 47, No. 1, p. 45
Penn State University moves ahead on spray irrigation project to demonstrate how constructed wetlands using compost in the soil media filters out nitrates.
Drew Mather

ABOUT TWO MILES from the main campus of Pennsylvania State University in State College, Pennsylvania, a land experiment to treat wastewater effluent continues today in much the same fashion in which it began in 1962. Known as the “Effluent Spray Irrigation Research Project”, this study formed the basis for a present day 516 acre site that has been affectionately called “The Living Filter”. This moniker comes from the site’s ability to filter out nutrients from the wastewater effluent using inherent soil properties and associative vegetation and microorganism assemblages. This “Filter” treats between 3 and 4 million gallons of wastewater daily.
More appropriately referred to as the Land Application Site (LAS) by its management agency, the University Area Joint Authority (UAJA), this site receives wastewater effluent from nearby Penn State University, a campus with current enrollment at over 41,000 students, and the surrounding city of State College, with a 2000 census population of over 38,000. Wastewater received at the LAS first undergoes primary and secondary treatment by the UAJA to remove solids and kill off the bacteria and viruses present in the effluent.
The LAS is located in Nittany Valley on an upland which consists of both cropland and forest. Karst topography dominates the landscape due to the underlying limestone bedrock. The differential weathering of the limestone bedrock produces an undulating, almost wavelike pattern to the overlying soils.
Soils on this site are remarkably deep (between 10 and 150 feet) and of mainly a silt loam texture. The soils in this area are an excellent filter media to hold nutrients in the wastewater in place until plants and other organisms can take them up and use them for growth. The filtered water eventually percolates through the deep soil profile and recharges the groundwater. Depending on the time of year, more or less water will percolate down into the soil and stay there for later use by plants and microorganisms.
A network of over 60 miles of pipes with 3,000 sprayheads applies approximately one-sixth of an inch (.4 cm) of wastewater per hour to the surrounding soils for a maximum application rate of two inches (5.08 cm) per week. Only about seven percent or 36 acres are spray irrigated at any one time. Crops such as cereal rye, wheat, corn, and soybeans are planted on the site and help to uptake nutrients present in the treated wastewater. Crops are harvested every fall and used for animal feed and animal bedding. No crops are currently being used for human consumption.
Due to the chemical makeup of some of the nutrients in the wastewater, the soil’s ability to “attract” these nutrients can be challenging. Nutrients such as nitrates are negatively charged in the same way one pole of a magnet is negatively charged. The negative and positive ends will attract, but two negatively charged ends will repel. Unfortunately, particles which make up what we refer to as soil – the sand, silt, clay, and organic matter – are mostly negatively charged. Because of this charge similarity, nitrates cannot readily “stick to” soil particles. Due to the charge similarity, these nitrates will go unfiltered through the soil column and enter the groundwater.
While nitrates are an important source of nitrogen for growing plant tissues, elevated levels of nitrates in drinking water (maximum contaminant levels set at 10mg/L NO3–N by the EPA) have been linked to a public health problem in humans known as methemoglobinemia or “blue baby syndrome.” Elevated nitrate levels discharged into watersheds can also lead to eutrophication, a process whereby excesses of nutrients like nitrogen and also phosphorus in aquatic systems stimulate the growth of algae and various plant life at the expense of other life forms. Fish kills are common in streams and lakes where eutrophication has occurred.
Although soils have a hard time “holding onto” the nitrates, plants and their respective root systems can absorb nitrates as well as phosphates to build plant tissue. During the spring and summer, when temperatures and sunlight are adequate to allow for rapid plant growth, plants can act as a major filter for uptake of these nutrients. In the fall and winter, when plants have lessening capacities to take up nitrates and phosphates, the situation changes and these nutrients can once again enter ground and surface water.
While excess nutrients sprayed on the LAS present management challenges, an Agricultural and Biological Engineering professor at Penn State, Dr. Robert Shannon, is currently utilizing less than a quarter acre of land on the same site to demonstrate how a constructed wetland partially made of compost can combat these challenges.
For regulatory purposes under the Clean Water Act, the term “wetlands” is defined as “those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs and similar areas.” Constructed wetlands, or “replacement wetlands,” are required by state and federal laws to replace the areas and functions of natural wetlands destroyed by highway and other building projects. Treatment wetlands are specifically designed and built to mimic and enhance the water quality functions of wetlands, and use the natural processes inherent in wetlands, such as those that are involved with wetland vegetation and their associative soils and microorganisms, to improve water quality.
In the case of treating wastewater effluent, constructed wetlands serve an important function of trapping and treating the effluent’s nutrients, making them available to plants and microorganisms for transformation into innocuous and nonpolluting substances.
The wetland project on the LAS came about due to concern over elevated levels of nitrate in water wells near the LAS and the surrounding watershed. The watershed includes Big Hollow Creek and Spring Creek, both used by a large number of residents in Centre County for recreation. A call for researchers to come up with innovative ways to treat excess nutrients in the spray irrigation runoff was made. Dr. Shannon answered the call.
The project has a laboratory or greenhouse component, and a field component. Charles Walker, one of Shannon’s former graduate students, incorporated the first component into his master’s thesis work. Walker used small plastic columns made of PVC that contained various mixtures of soil media. These columns, or “mesocosms” as he referred to them in his thesis, were devised to test the ability of the media to remove excess nutrients from a nutrient enriched water that simulated surface water runoff at LAS (20 mg/L of nitrates (NO3-N) and 10 mg/L of phosphates (PO43-P)). The soil media consisted of mineral soil from the LAS (mainly Hagerstown Silt Loam) and composted cow manure and soybean fodder in various mixtures. The compost had a C:N ratio of 9. The mineral soil had a C:N ratio of 13. Treatment regimens consisted of 3 replicates each of 0, 5, 10, and 20 percent compost to mineral soil for a total of 12. The percentages were based on dry mass of both the compost and the mineral soil.
Results from Walker’s study showed average nitrate removal rates in the columns significantly increased as compost percentage in the columns increased. Nitrate removal rate constants nearly doubled (from 1.36 d-1 to 2.69 d-1) when mesocosm soils were amended with 20 percent compost. Phosphate removal percentages in the compost-amended soil media were not as dramatic as the nitrate removal rates. The highest phosphate removal percentages were found in the zero percent compost-amended soil media at 62 percent and the lowest removal percentages were in the 20 percent compost-amended soil media at 21 percent. The five percent compost-amended soil media had a phosphate removal percentage of 37 percent. According to Shannon, these results may be due to the presence of more phosphorus binding sites present in the clays of the mineral soil with no compost added. Despite the differences in how phosphates and nitrates responded to the different compost-amended media, both nutrients were adequately removed after passing through the soil columns.
With promising results from the greenhouse study, the second phase of the project could proceed. Construction of the wetlands began in the fall of 2002 and was completed during the spring of 2003. A stone centered waterway was constructed to channel runoff into the wetland (Figure 1). A six inch tee section was devised to provide for even distribution of the wastewater. The entire site was excavated keeping all the topsoil for later mixing with the compost. Berms made of the mineral soil and compost mix were created in a serpentine design and also at the back of the depressional area of the wetland site to help control the flow of water once it arrived in the experimental area. The 20 percent compost to mineral soil mixture was chosen for the field component of the study due to its impressive ability to remove nitrates from the greenhouse study. The soil media in the field was identical to the media used in Walker’s mesocosm study.
The soil media was mixed and applied at a depth of 1.5 feet atop a clay liner. Drainage pipe and laterals were added throughout the constructed wetland prior to the application to channel percolated water to an outflow unit called an AgriDrain water level control structure (Figure 2). The structure was fitted with an automated water sampler so that water exiting the constructed wetland site could be carefully controlled and tested for nutrients. Inflow water samples were also collected and analyzed for nutrient loads. A facultative wetland species seed mix (dominant species Bidens cernua or “Beggarticks” in common parlance) was broadcast over the mixed media and allowed to establish.
The field component of this project is still ongoing and results are preliminary and unpublished. Early findings show similarity to the greenhouse component in terms of nitrate removal rates. For one irrigation runoff event, the peak inflow nitrate load of approximately 2200 mg of nitrate N per minute was reduced to approximately 600 mg of nitrate N per minute in the outflow, resulting in significant nitrogen removal within the wetland. In terms of cumulative loads of nitrate N over a 24 hour period, the constructed wetland held onto 690 g of nitrate N resulting in a 65 percent reduction of nitrate N leaving the site as outflow (Figures 3 and 4). Similar nitrogen removal was demonstrated in other monitored runoff events. The implications are very positive for the use of such constructed wetlands using compost to treat wastewater effluent.
While Shannon is optimistic about these preliminary results, he adds that there is still much work to be done in terms of determining the ultimate fate of the stored up nitrates and phosphates in the substrates of the constructed wetlands. Shallow groundwater monitoring wells were recently installed at the site, and students have been assigned to monitor spikes in nitrates or phosphates entering the groundwater table. The information gleaned from these studies will lend further knowledge of how well this substrate and the entire wetland are treating the influx of excess nutrients. Shannon adds that determining microbial denitrification (turning nitrates back into harmless nitrogen gas) rates would also be beneficial and hopes that some studies can be done to monitor this very important microbial activity.
In terms of future studies with compost-amended soils used in constructed wetlands, Shannon would like to see more research on how soil amendments remove excess nutrients in wetlands. Information on how soil amendments affect long term vegetation establishment in replacement wetlands would benefit public agencies regulating and monitoring wetlands, as well as to firms designing and building them. Finally, a project designed to answer which types of compost, such as those of differing feedstocks and ages, perform better in terms of removing excess nutrients best also would be constructive.
Drew Mather is in the Crop and Soil Sciences Department at Penn State University. He can be contacted via e-mail at dmather@psu.edu.

Sign up