Silage leachate is one of the most notorious wastewaters produced in agricultural procedures. It has a high acidity (low pH) and low reduction potential (pE). In this study, we aimed to treat silage leachate. 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 the application of limestone in anaerobic filters would increase pH and the condition would be ideal for methanogens and bioremediation of the leachate. The pH increased in relatively short retention times and thereby, improved the corrosiveness of the silage leachate. Also, more than 95 % of both reactive and total phosphorus was removed. However, the removal of COD countered with the high ammonium concentrations which transformed to ammonia at the increased pH. Nevertheless, these bioreactors proved successful when the ammoniacal N concentrations were low. In such cases, the anaerobic bioreactors removed more than 50% of the COD.
In contrast to the limestone-bed reactors, the Continuous stirred-tank reactors (CSTR) was successful in removing the COD from the leachate. The leachate was gradually added to these reactors filled with anaerobic sludge, thus, controlled the concentration of ammonia. When the production of biogas was observed, a shock was introduced to the system by adding 50 % leachate. These reactors removed 30 to 50% of the COD in residence times of 24 and 48 h, respectively which reduced the COD from 20500 to 8000 mg/L.
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. In spite of the small volume, it has 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 COD 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”.
The fermentation of silage in the bunkers produces 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 has focused on the management of silos, for example through controlling its moisture, 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, decreasing its COD and nutrient content. Therefore, here we tested the application of digestion filters (with limestone) and continuous stirred-tank reactors in the removal of COD. We also tested the effect of activated carbon on the COD reduction.
1- Field experiments
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 dairy farms. The first series was taken from a 1000 gallons underground tank used for the silage leachate storage in Harford Cornell dairy research center. The leachate in the pit could have been mixed with rainwater.
The second series was acquired from a farm in the Tompkins’s County area. 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 had not precipitated for 4 days so that 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 the phenate method. Chemical Oxygen Demand (COD) was measured using the colorimetric method (CHEMetrics, Inc.). The pH of the samples was also measured. The gas samples are to be analyzed with a gas chromatograph.
In the anaerobic Continuous stirred-tank reactors (CSTR) essays, 3 ml silage leachate obtained from the Tomkins County farm was added to 100 ml of sludge obtained from the Ithaca Wastewater treatment plant. The essays were put on the magnetic stirrer for constant mixing and were kept at 28 ° C (Figure 2). Treatments included essays with and without activated carbon. One gram activated carbon was added to the essays to simulate a moving bed bioreactor where the activated carbon can provide a desirable surface for biomass. The chemical oxygen demand of the sludge and the leachate was 28600 and 20500 mg/L, respectively.
The essays were sealed, and the air was replaced with nitrogen gas to provide an anaerobic condition for anaerobic digestion and the growth of methanogens. In three consecutive days after that, 5ml more leachate was added to the system each day. After the production of biogas was observed, on the 4th day, 50 ml of the mixture of sludge and leachate was drawn from the anaerobic essays and was replaced with 50 ml leachate. At this point, the COD concentrations or the initial COD concentration of the essays varied from 13300 to 17800 mg/L.
1- Field experiments
The result of the field experiment showed 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 3).
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 (Figure 4a). For the second series, however, the removal occurred at a very slow rate. Only 23% removal of COD was observed in 30 days (Figure 4b). The slow pace of the removal of COD was attributed to the high ammonical 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, controlling the effect of NH3 was considered.
In a laboratory experiment, we tested the CSTR design for silage leachate treatment. In this experiment, we gradually added the leachate to the bioreactors filled with sludge. After assuring that inoculums were in full play, we introduced a shock to the system by adding 50% leachate. The COD concentrations at the time of the start of the shock (adding 50 ml of leachate to the system) varied from 13300 to 17800 mg/L. Both these values are less than the initial COD concentrations of the leachate (20500 mg/L) and the sludge (28600 mg/L). This reduction in COD and the observed production of biogas indicated that the reactors were conditioned and bacterial growth occurred. After the shock, at the residence times of 24 h and 48h, and the average COD concentrations were 11600 and 8000 mg/L, respectively (Figure 5a). The removal of COD was 25 ± 1% at the residence time of 24h and 53 ± 3% at the residence time of 48 h for the essays without activated carbon (Figure 5b). In the essays with the activated carbon, the corresponding values were 29 ± 16 % and 45 ± 13 % (Figure 5b). It is worthy of note that the difference between COD concentrations and removal was not significant between the essays with activated carbon and those without it. In fact, in reactors with activated carbon, the COD concentration and removal was more variable.
This study indicated that limestone increased the pH to 7.5 of the silage leachate and thereby, reduced its corrosiveness. The application of limestone also proved to be successful in the removal of more than 95% of the reactive and total phosphorus from the leachate. However, use of the anaerobic digester with limestone did not decrease the COD of the leachate, due to the high ammoniacal N concentrations. The observed ammonical N concentrations were likely toxic in anaerobic digesters at the increased. Therefore, measures must be taken to mitigate the effect of ammonia (NH3) on the leachate treatment. Also, We also tested the application of the anaerobic filter with three layers of Raschig ring, limestone chips, and activated carbon. The filters were clogged shortly after use. That led us to pursue the application of continuous stirred-tank reactor design. The application of the CSTR design was successful in reducing the COD concentration of silage leachate. One of the ways to keep concentrations of ammonia under control is to keep the pH at levels below 7. The gradual addition of the leachate assured that the pH would increase slightly but not to the extent where the transformation of ammonium to ammonia could inhibit the microbial growth. Such an approach proved to be preferable to that of the anaerobic filter or limestone beds, where the COD failed to decrease. The CSTR removed 25 and 50 % of the COD in 24h and 48h, respectively. The next step for these experiments is to test the CSTR design in a larger scale and monitor its performance in longer residence time.
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 this research, we visited many different farms and talked to many farmers on various occasions on the phone or otherwise in person. This research promoted communication with local farmers and provided us with the opportunity to observe the problems that farms deal with especially those related to the production of the silage leachate. The damages the concrete structure on its route (Figure 4) can add to the costs of the maintenance of the farms. Also, we have been contacted by the farmers whose storage lagoons overflowed for which they face penalties. We are working hard to be able to improve the leachates water quality through cost-effective methods.
I also was able to make a comparison between the management strategies that are taken by the farmers. In Harford dairy research farm which is equipped with machines to wrap the silo in the bags, there was little leachate produced from the fermentation (Figure 7). The wrapping of the silo provides better anaerobic conditions for fermentation. 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.