Silage leachate is one of the most potent wastewaters produced in agricultural procedures. It has a high acidity (low pH) and low reduction potential (pE). In this study, we aimed at the treatment of silage leachate using limestone bed bioreactors. There were three different factors to consider; pH, nutrients such as nitrogen and phosphorus, and the chemical oxygen demand (COD). Our initial hypothesis was that with increasing pH, the condition would be ideal for methanogens and bioremediation of the leachate. To date, our results showed that limestone bed bioreactors successfully increased pH in relatively short retention times and thereby, improved the corrosiveness of the silage leachate. In addition, with increasing pH, limestone precipitated phosphorus. More than 95 % of the reactive phosphorus and total phosphorus was removed. However, the removal of COD and ammonium (ammonia at higher pH) were intertwined. On the one hand, the removal of COD was required so that the redox condition would be agreeable for the oxidation of ammonia to nitrate. On the other hand, the excess ammonia concentration was toxic to microbes which introduced an unprecedented twist to the efficiency of the bioreactors. Our one leachate source contained 2 to 10 times greater ammoniacal N concentrations than those observed in previous studies. Therefore, an additional step should take place to eliminate ammonia. Nevertheless, these bioreactors proved successful when the ammoniacal N concentrations were low. In that case, the bioreactors removed more than 50% of the COD.
The fermentation of silage in the bunkers produces a leachate which has a low pH (high acidity) and a low pE (low reduction potential) and is rich in nutrients and organic matter. Silage leachate is among the most potent wastewaters. Several studies observed that BOD and COD exceeded 49000 mg L-1 (Gebrehanna et al., 2014). These characteristics pose a danger to the environment and infrastructure of the farms. The corrosive runoff from the silage bunkers dissolves the concrete and burns the vegetation along its route. The direct discharge of the silo’s leachate to the water bodies, cause an immediate drop in the dissolved oxygen since it is comprised of readily available organic matter such as lactic acid. Only1 L of silo leachate depletes the dissolved oxygen (DO) of 10,000 L of water below the tolerance of fish (Gebrehanna et al., 2014). The seepage of the silage leachate into freshwater bodies raise many concerns in the past. Fish-kill in large scales was reported in streams in NY state due to the leakage of the silo leachate, and the resulting poor water quality (Schade, 2005). Also, the extreme nutrient concentration is a concern for the freshwater bodies, especially the lakes in the northeast US. The nutrients from such sources cause increased algal growth which strips away the dissolved oxygen and thereby leads to eutrophication in ponds and lakes. Lake Champlain, in upstate NY, provides an appropriate example of the consequences of such nutrient concentrations in the northeastern US. In this lake, the excess amount of phosphorus cause unsightly summer blooms of blue-green algae which release noxious odors and toxins that pose health risks including gastrointestinal issues and skin irritation(LCBP, 2008; Morse and Munroe, 2011). Given the extremely poor quality of the leachate, federal and state regulatory provisions were taken to ban the discharge of silage bunker leachate from the concentrated animal feeding operations (CAFOs) into streams and waterways (NYSDEC, 2017). Also, farmers frequently face fees and fines because of the disposal of insufficiently treated silage leachate. The current safety measures require the silage leachate to either be stored, if low flow occurs, or otherwise for the wastewater to be conveyed to vegetated buffer areas (NRCS, 2013). Small farms, however, do not have storage lagoons.
While most research focuses on the management of silos, few studies investigated the treatment of the silage leachate. Anaerobic digestion of the silage leachate was investigated previously by adding limestone chips to digesters in a laboratory experiment (Barry and Colleran, 1982). By increasing the pH of the silage leachate to 8, they observed a COD reduction of 86 to 89 % in 3 days. In the current study, we plan to investigate the treatment of silage leachate using limestone bed bioreactors. This method includes applying limestone for increasing the pH so that an ideal environment for anaerobic digestion is provided. In addition, we will monitor the nutrient removal.
Our objective is to improve the quality of the silage leachate. In comparison to other agricultural wastewaters, silage bunker leachate has the poorest water quality. Though small in volume, it has a great potential for environmental contamination. This is due to the combined low pH (high acidity), low pE (high electron activity), and high nutrient content, which is unique among wastewaters. Given these facts, this research will target the following objectives in silage leachate treatment:
1- To remove BOD from the silage leachate
The pH for the silage bunker leachate ranges from 3.6 to 5.5 (Cropper and DuPoldt, 1995)), far from optimal biological pH, therefore, the current natural removal techniques will not be efficient or sufficient. The optimum pH for anaerobic systems ranges from 6.5 to 7.5, depending on the organic matter content (Liu et al., 2008). This pH can, however, be achieved by flowing the wastewater through lime bed bioreactor (Barry and Colleran, 1982).
2- To determine an optimum hydraulic retention time
Our second goal is to find the relationship between BOD removal and hydraulic retention time (HRT) in the bioreactors. Together with the estimated flow rate from (Cropper and DuPoldt, 1995; NRCS, 2013), this information will provide guidelines to design bioreactor in field settings. To reduce the cost of the constructing such bioreactors, it is important that the size is not overestimated.
3- Nutrient removal
This project will investigate the removal of nutrients, specifically P and N, from the wastewater. By increasing pH and the abundance of Ca, P can precipitate out of the solution. Nitrogen (N) will be removed in an anaerobic bioreactor.
The ultimate objective of this research is to help farmers to avoid fees and penalties by providing an approach to treat the silage leachate, and thereby, reduce the farm’s spending. In general, the current research will address three of five required themes: “the reduction of environmental risks”, “the reduction of costs”, and “the conservation of soil, the improvement of water quality and the protection of natural resources”.
Initially, the silage leachate was brought from the field to the lab for the flow-through bioreactors. However, the tubing used for the lab bioreactors clogged by the suspended organics in the leachate. Therefore, the experiments were done in the field. Six batch bioreactors using 208 L plastic drums were constructed in Harford dairy research farm, Cornell University (Figure 1). Two outlets with faucets were placed on the side, for sampling, and at the bottom, for emptying the barrel when needed. The bioreactors were sealed on the top. The venting occurred through a bottle filled with water so that oxygen would not enter the bioreactors. The gas samples were taken through septa using syringes and needles. Three of the bioreactors were filled with limestone, and the three other bioreactors were used as controls. One ton of limestone chips was used to fill the three barrels. The average size of the limestone chips was 1 cm2.
The bioreactors were filled with two batches of the leachate acquired from two different fields. The first series was taken from a 1000 gallons underground tank used for the silage leachate storage in Cornell dairy research center at Harford dairy farm. The pit water could have been mixed with the rainwater and had odors such that of hydrogen sulfide. The second series was acquired from a farm in the Tompkins’s County area. In this farm, the silage was stored in the bunkers equipped with tile drains around them collecting the leachate in a 1000 gallon tank. The leachate was taken when it did not precipitate for 4 days so that a dilution with the rainwater did not occur. After the bioreactors were filled with the leachate, daily samples were taken using propylene centrifuge tubes. The water temperature was recorded when sampling. The samples were analyzed for total phosphorus (P) and reactive phosphorus content using the ascorbic acid method. Ammoniacal N concentrations were measured using phenol method. Chemical Oxygen Demand (COD) was measured using colorimetric method (CHEMetrics, Inc.). The pH of the samples was also measured. The gas samples are to be analyzed with a gas chromatograph.
The result of the experiments indicated that limestone bed bioreactors successfully removed phosphorus from leachate. In the first series of experiments, the pH increased from 6.1 to 7.5. The pH for the second series rose from 4.3 to 7.5. In the two batches, total P and reactive P remained constant in control treatments, however, a decrease in both was observed in limestone bed bioreactors. The first batch saw a reduction of the reactive P from 30 mg/L to 3 mg/L. The total P in this series decreased from 235 mg/L to 10 mg/L (Figure 2).
A similar observation was made in the second batch of the experiments. The reactive P and total P decreased to 4 and 19 mg/L from 514 and 400 mg/L, respectively. Dissolution of organic phosphorus may have occurred in the control treatment during the experiment since an increase in the reactive P was observed after 7 days from the start. Overall, more than 95% of the reactive and total phosphorus was removed from the leachate by the bioreactors.
Although the bioreactors were successful in the P removal, they underperformed in the chemical oxygen demand (COD) removal. For the first batch, bioreactors removed 54 % of the COD in 10 days, declining the COD from 3900 to 1700 mg/L. For the second series, however, the removal occurred at a very slow rate. Only 23% removal of COD was observed in 30 days. The slow pace of the removal of COD was attributed to the high ammoniacal N concentration. Due to a higher pH achieved by the addition of limestone, ammonium ion (NH4+) transformed to ammonia (NH3) which is toxic to the microbes. The ammoniacal N concentrations observed in the first series and the second series were 300 and 1600 mg/L, respectively. These values, especially the latter has proved to be toxic to the anaerobic digestion process by other studies (Yenigün and Demirel, 2013; Sung and Liu, 2003).
This result indicated the occurrence of a complication due to the excess ammoniacal N for the treatment of silage leachate which is rich in nutrients. To pursue this research forward, the removal of ammonium should be taken into consideration. One of the reasonable cost-effective ways to remove ammonia in anaerobic digesters is the application of zeolites (Wang and Peng, 2010; Krakat et al. 2017). More experiments will be carried out to investigate the effect of zeolite on the adsorption of ammonium, pH and the removal of COD.
This study indicated that limestone increased the pH of the silage leachate and thereby, improved its corrosiveness. The pH of the two batches increased to 7.5. This has a great impact on the preservation of the concrete structures on the leachate’s route on the farm. The application of limestone also proved to be successful in the removal of reactive and total phosphorus from the leachate. More than 95% of phosphorus precipitated in the bioreactors.
However, the limestone alone, cannot decrease the COD of the leachate, due to the high ammoniacal N concentrations. The observed ammonical N concentrations are toxic in anaerobic digesters. Therefore, measures must be taken to primarily remove the ammonium from the leachate. This will be a deviation from our initial plan to remove the COD and consequently, remove ammonium by oxidation. As mentioned in the previous sections, our plan for the future experiments includes the removal of ammonium by adsorption.
Education & Outreach Activities and Participation Summary
The result of this research will be beneficial for the farmers who face difficulties in disposing of the silage leachate. Therefore, farmers will be targeted as the primary addressees. We will present our results in the northeast certified crop advisor’s (NECCA) meetings. Besides, the result of this research will be presented at local, national and international conferences such as ASABE. Therefore, farm owners and engineers and designers will be aware of the results of the project. We also will submit a manuscript to a peer-reviewed journal and develop an extension fact sheet for soil and water lab website at Cornell University(soilandwater.bee.cornell.edu).
The objective of this study is to improve the quality of the silage leachate. In comparison to other agricultural wastewaters, silage bunker leachate has the poorest water quality. Though small in volume, it has a great potential for environmental contamination. This is due to the combined low pH (high acidity), low pE (high electron activity), and high nutrient content, which is unique among wastewaters. Addition of such waster to the soil can cause the release of the heavy metals. Silage leachate direct discharge to the stream cause a depletion of the oxygen. Also, its containing nutrients pose a danger to the downstream water quality. Therefore, treating it would reduce the environmental risks and helps to conserve the soil. Moreover, some farmers will have to deal with fees and fines if the disposal of such leachate is not properly done on their farm. Application of a system for treating it will help reduce such fees.
During the course of this research, I visited four farms. I also talked to many farmers on different occasions on the phone or otherwise in person. This research promoted my communication skills with the farmers and provided me with the opportunity to see the problems that farms deal with especially related to that of the production of the silage leachate. I observed how silage leachate damages the concrete structure on its route (Figure 4) and how this adds to the costs of the maintenance of the farms.
I also was able to make a comparison between the management strategies that are taken by the farmers. In Harford dairy research farm, equipment was purchased to wrap the silo in the bags (Figure 5). With this method, little silage leachate was produced. The amount was so small that in the peak season, leachate was not enough for us to fill the barrels. According to the manager, the fermentation occurs more efficiently in the bags since the anaerobic environment is provided more efficiently. The quality of the silo is also finer at the end. Whereas, in the farmers’ field where silo was stored in the bunkers, more runoff was produced. One must note that purchasing expensive equipment is not feasible for all the farmers. Therefore, the considerable production of silage leachate occurs in silo bunkers. For this reason, it is important to find a cost-effective way to treat silage leachate to contain its environmental impact in the farm and beyond it. I was asked by the farmers when such treatment system would be applicable on-site. This has certainly upgraded my motivation me to pursue this research. Managing agricultural wastewater quality not only is my research and profession but also my passion.