Cost benefit assessment of a novel thermal stripping – acid absorption process for ammonia recovery from anaerobically digested dairy manure

Liquid dairy manure, a mixture of as-excreted manure, used bedding and wastewater, is rich in nutrients and typically applied to cropland as an organic fertilizer (USDA 2009; ASABE 2010). Many dairy farms, especially large-size operations, generate excess liquid manure that cannot be applied to their own land, on the basis of nutrient application rates. Anaerobic digestion has been increasingly applied to stabilize liquid manure and produce biogas for energy production. However, it converts organic nitrogen to bioavailable ammonia. Because more nitrogen is bioavailable in anaerobically digested dairy manure than in undigested manure, land application rates must be reduced for digested manure and any excess digested manure must be treated further or transported off-site (Nkoa 2014). To facilitate treatment and beneficial use, digester effluent (digestate) is mostly separated into liquid and solid fractions (Fuchs & Drosg 2013). Solid-liquid separation and subsequent treatment, including ammonia removal from the liquid fraction, often adds financial burden to the operation of anaerobic digesters.

Despite steadily rising costs and more stringent regulations for land application, a digestate processing plant is economically viable only in a small number of cases, and the search for improved and more economic approaches continues (Fuchs & Drosg 2013). Recently, instead of removing ammonia, Tao & Ukwuani (2015) developed a technology that couples thermal stripping and acid absorption to produce ammonium sulfate granules from digestate. Aqueous ammonia in digestate is converted to free ammonia by boiling (about 101 °C) at pH 9, which is stripped out of the digestate and absorbed in dilute sulfuric acid, to form ammonium sulfate. When the acid solution is oversaturated, ammonium sulfate granules are produced, which can be sold commercially.

When the ammonia recovery process is applied to an anaerobic digester recirculation line, the need to heat the digesters is eliminated or reduced. Therefore, the integration of ammonia recovery into anaerobic digestion could make energy-intensive thermal stripping viable. Moreover, thermal stripping has several design and operational advantages over what is, currently, the most promising air-stripping ammonia recovery process for manure digestate (Tao & Ukwuani 2015). This paper analyzes cost inputs, cost savings, and potential revenues of ammonia recovery from anaerobically digested dairy manure at a large dairy farm (Twin Birch Dairy LLC), assesses net present value (NPV) and benefit/cost ratio, and explores the sensitivity of both NPV and benefit/cost ratio to various market uncertainties. Cost-benefit analysis helps gauge the most economic form of thermal stripping – acid absorption and allows comparison with other ammonia recovery methods.

Twin Birch Dairy is a dairy farm in central New York State, USA, with 1,100 milking cows. As-excreted manure and used bedding are scraped off the barns' concrete floors, mixed with milking washwater, and treated in two plug-flow anaerobic digesters at mesophilic temperatures (37 °C on average). A combined heat and power system uses the biogas to heat the liquid manure fed to the anaerobic digesters. The digesters are operated in parallel and have a total liquid manure loading rate of 110,000 L/d. Digester effluent passes through a screw press separator. The separated solids are composted, and used for bedding or sold. The separated liquid is stored 2.3 km away and used to irrigate the farm's cropland, which comprises 1,012 hectares of wheat, corn and alfalfa. Most of the cropland – 890 to 1,012 hectares – was irrigated with the separated liquid until 2015.

Because of the shorter hydraulic retention time in the digesters and the enhanced hydrolysis of organic matter due to treatment during thermal stripping under options 2 and 3, relative to Option 1, it is conservatively assumed that the ammonification rate under options 2 and 3 will be the same as that achieved presently – i.e., Option 1.

When equipping three stripping vessels with one absorption column for batch operation, it is estimated that each column would be of 0.5 m diameter and 0.8 m height, with a total capacity of 157 L, and hold 98 L of acid solution. Four columns will be required for Option 2 and two for Option 3. Carbon steel or polyethylene columns are typically used to hold sulfuric acid solution. It was proposed to assemble ammonia absorption columns with conical, high-density polyethylene tanks. The retail price is approximately $150 each.

With thermal stripping at pH 9 in an anaerobic digestion recirculation line, the digester effluent pH will increase from 7.8 by less than 0.06. Laboratory experiments showed that increasing the digestate pH from 7.8 to 9 for ammonia recovery would require 1.33 g-CaO/L-digestate. The unit cost of lime (CaO) is $70/tonne (Jiang et al. 2014). The cost of pH elevation can then be calculated using an equation similar to Equation (7).

The labor required to operate the ammonia recovery system varies with the numbers of stripping vessels and acid absorption columns, which depends on the recirculation rate. Each stripping vessel will operate for 8 batches per day. Each batch operation will last approximately 3 hours, and all stripping vessels and absorption columns will operate in parallel. It is hence estimated that one operator per 8-hour shift will be able to run the stripping vessels (11) and absorption columns (4) in Option 2, including feeding and discharging the stripping vessels, harvesting the ammonium sulfate granules and renewing the sulfuric acid solution. Option 3 will only need one, half-time operator. An hourly wage of $17.94 was used – the national mean hourly wage in 2014 for farm equipment mechanics and service technicians, as reported by the U.S. Department of Labor (BLS 2015b).

Maintenance and repairs are considered contingency costs. They may arise due to requirements like vessel and pipe cleaning, unclogging, machine oiling, and parts replacement. To secure efficient performance and avoid unnecessary breakdowns, annual maintenance costs were estimated at 5% of total capital costs (Sinnott & Towler 2009).

With ammonia recovery, the ammonia concentration of the digester effluent will decrease (Figure 1). As a result, a dairy farm may apply more digested manure to its own land with less extra manure to be transported to other farms. In New York State, tankers are used to transport and spread liquid manure. In a survey of dairy farms in the state, Howland & Karszes (2014) reported an average liquid manure application cost of $2.99/m³. The total included (1) operating costs (labor, fuel, utilities, and repairs); (2) ownership costs (depreciation, interest and insurance); and (3) the costs of custom service provided by an off-farm company.

Howland & Karszes (2014) did not find a significant relationship between liquid manure application cost and distance to field, between 0.7 and 16 km. Twin Birch Dairy sells the separated solids and stores the separated liquid 2.3 km away for irrigation within a further 5–6 km. The compliance cost used was, therefore, estimated by multiplying the volume of digestate to be transported off-site by this unit manure application cost. The latter is lower than, but in the same order of magnitude as, that in Europe for digestate disposal (Fuchs & Drosg 2013). Based on the ammonia concentrations of digester effluent under the three options (Figure 1), it is estimated that Option 1 will need storage for 30,694 L/d more digestate off-site and Option 3 13,462 L/d more, compared to Option 2.

In addition to the production of ammonium sulfate granules, the thermal stripping of ammonia may enhance anaerobic digestion due to thermal treatment of particulate organic matter in the recirculated digestate and zero risk of ammonia toxicity to methanogens in the digesters (Tao & Ukwuani 2015). The synergistic effects of integrating ammonia recovery to anaerobic digestion have yet to be quantified. The economic analysis in this study does not take the indirect effects of ammonia recovery on anaerobic digestion into account.

Ammonium sulfate production depends on both the recirculation rate and the digester effluent ammonia concentration (Figure 1). During each 3-hour batch operation, almost all ammonia in the digester effluent could be recovered and it is estimated that ammonium sulfate productivity would be 169.6 kg/d and 95.2 kg/d, respectively, in options 2 and 3.

A heat balance shows that the anaerobic digesters will attain the digestion temperature of 37 °C when the ammonia-recovered hot manure is recycled to the inlet of the digesters at the recirculation rate of Option 2, thus saving the cost of energy for digester heating at K_e E_q. The savings can be calculated for Option 3, with its lower recirculation rate, similarly.

The NPV and benefit/cost ratio are calculated without consideration of a separate inflation rate. The costs and returns are estimated using current unit costs and revenues instead of predicted prices. Instead, sensitivity analysis was carried out to evaluate the uncertainties associated with the key cost and benefit components.

Capital and operating costs, and revenues were calculated for each ammonia recovery option, using the cost and revenue models above, as well as the design and operating parameters. As summarized in Table 1, options 2 and 3 have similarly high benefit/cost ratios. However, the NPV of Option 2 is 1.85 times that of Option 3, while that for Option 1 is negative. Options 2 and 3 both require small amounts of initial investment so, Option 2 is economically better. In addition, options 2 and 3 will create 3 and 1.5 jobs, respectively, operating the ammonia recovery system 24 hours a day. Labor cost is the largest component of the operating cost, accounting for between 61 and 69% of its total.

The specific benefit of thermal stripping – acid absorption under Option 2 is $0.018/L-digestate, $3.25/kg-(NH₄)₂SO₄, or $0.50/d/cow. The specific operating cost of the process, excluding labor, under Option 2 is $0.006/L-digestate or $1.12/kg-(NH₄)₂SO_4. This is lower than the optimized values for gas stripping – acid absorption, i.e., $0.014/L-digestate or $1.63/kg-(NH₄)₂SO₄ (Jiang et al. 2014).

The price of sulfuric acid was between $21 and $141/tonne between 2009 and 2014 (ICIS 2014). This level of price variation relative to current prices ($55/tonne) will not result in any significant changes in NPV or benefit/cost ratio because it only accounts for 1.1% of total operating costs (Table 1). The net energy cost – i.e., total energy cost minus savings in digester heating – is only 5.7% of the labor cost for Option 2, so, the sensitivity analysis on the cost components was carried out only in relation to potential changes in labor cost. The hourly wage for farm equipment mechanics and service technicians in the USA varied in 2013, being $11.18 at 10% percentile, $13.86 at 25%, $17.38 at 50%, $21.55 at 75%, and $25.88 at 90% (BLS 2015b). With these variations, the NPV and benefit/cost ratio responded as shown in Figure 3. The NPV will be zero and the benefit/cost ratio 1 at a wage rate of $40.65/h (Figure 3). As indicated by a greater slope of change, the NPV is more sensitive to the ammonium sulfate price than to wage rates. However, it appears that the benefit/cost ratio is equally sensitive to both factors. The hourly wage will probably not reach $41, so, ammonia recovery via thermal stripping – acid absorption will make a profit when ammonium sulfate granules can be sold for $2.77/kg-(NH₄)₂SO₄ or more.

Ammonia recovery via thermal stripping – acid absorption is estimated to be profitable. Although drawing digester effluent at 14 and 28% of the manure loading rate for ammonia recovery yields similar benefit/cost ratios, the higher recirculation rate produces a higher NPV and creates more jobs.

Labor costs account for between 61 and 69% of total operating costs for thermal stripping – acid absorption. Excluding these, the specific operating cost of thermal stripping is substantially lower than that of air-stripping.

The uncertainties in NPV and benefit/cost ratio are mainly associated with the sale price of ammonium sulfate and the hourly wage rate.

This study was funded by a U.S. EPA grant to Dr. Tao (SU835723). The views expressed are solely those of the authors and do not necessarily reflect those of the Agency. We would like to thank the U.S. Department of State for a Fulbright Foreign Student Program Scholarship to Sohaib Anwar.