The concept of reducing slurry pH to decrease ammonia emission relies on the equilibrium between NH₄⁺(aq) dissolved in slurry and NH₃(aq) (Fangueiro et al., 2015). When acidifying slurry to pH 5.5, the relative acid content is modified and 99.98% is NH₄⁺ (Fangueiro et al., 2015).

An example of the relative share of ammonia and ammonium as a function of pH is illustrated by the following figure:

Figure according Kelly-Edwards (2018).

Normally, sulphuric acid H₂SO₄ is used as acidifying agent due to its cheap price and the fact that it on a weight basis contains about one third sulphur (32.7%), which replace otherwise needed fertilisation with sulphur. Joubin (2018) analysed the feasibility of other acids for slurry acidification, including formic acid, acetic acid, lactic acid, oxalic acid and nitric acid, and found that using other acids is possible, but the price for the acid consumption is up to two times higher than that of sulphuric acid. There is an interest for finding solutions for so-called bio-acidification, for instance by use of organic material with high sugar content, which would make in-house acidification technologies feasible for use in organic livestock production, which does not allow the use of sulphuric acid.

The amount of sulphuric acid needed for lowering the pH to the envisaged level depends on the initial pH value, the target pH value and the buffer capacity of the liquid livestock manure.

The acidification unit to treat the slurry, consists of the following main components:

• valve pit,

• process tank, and

• acid tank.

The slurry is typically running from the slurry channels in the stables by gravitation forces via tubes into a treatment tank; acid is added under stirring to reach a fixed pH level. Aeration is performed simultaneously to avoid foaming. Part of the slurry is returned to the slurry channels inside the stables to ensure deposited droppings are encapsulated in already acidified slurry, and the surplus is pumped to a storage tank. Treatment frequency depends on the slurry pH measured before each treatment, meaning the frequency increases with increasing initial pH. Normally, all the slurry in a herd will be treated 1-3 times daily. All processes are controlled and monitored automatically.

The Danish Environmental Protection Agency's Technology List sets the condition for using in-house acidification as a measure for reducing the ammonia emission, that pH of the slurry that returns from the stables on a monthly average basis does not exceed pH 6.0, and that the acidificed slurry never have a pH above 6.5. This is achieved by acidifying the slurry to pH 5.5 repeatedly. Furthermore, formation of foam in the slurry becomes a problem for pH-levels above 6.0 (Personal information, Kurt West, JH Agro A/S, June 2020).

The buffering capacity of slurry permits the pH to come back at its initial pH level after acidification. Indeed, a 60
day study of pH after different slurries were acidified to pH 5.5 showed the pH increased to pH 6.4 between ten and twenty days after acidification for different acids (sulfuric acid, acetic acid, citric acid, lactic acid) (Regueiro et al., 2016). According to Peterson et al. (2012), this rise of pH is due to: microbial activity and hydrolysis of volatile fatty acids, the mineralization of organic nitrogen and dissolution of carbonates. For in-house systems at start of the storage period, the target pH value is 5.5, and the slurry has at that stage been repeatedly acidified to pH 5.5. Fangueiro et al. (2014) considers in-house acidification a long-term acidification.

End/by-product 1 - explanation: The end product is acidified slurry. It has typically a pH of 5.5 and a higher sulfate content than untreated slurry.

Effects on air (emissions):

In-house acidification reduces gaseous emisssions of ammonia (NH₃), methane (CH₄) and other gases, both in stables, during storage and in connection to field spreading:

• Ammonia: The Danish Environmental Protection Agency's Technology List provides direct information on the recognized and verified reduction of ammonia emissions as a result of the use of in-house acidification, which is 64% for pig stables and 50% for cattle stables. Standard values for emission of ammonia from the barn and storage are for Denmark provided in Standard Values for Livestock Manure (Lund et al., 2019). The ALFAM2 model shows the loss of N as ammonia emissions by evaporation during slurry application, althoug the model does not include slurry acidified to pH 5.5, but alone to pH 6.0. The recognised emission reduction for pig stables is based on a VERA Verification from 2016. Several other studies have similarly reported considerable effect of in-house acidification on ammonia emission reduction. A Danish study concluded that frequent adjustment of the pH of pig slurry in a pig house (fattening pigs) with 1/3 drained floor and 2/3 slats reduced ammonia volatilization by 70% (Pedersen, 2004). From slurry storage tanks, Kai et al. (2008) estimated losses from acidified slurry being less than 20% of the emission from an untreated uncovered storage facility, and from fields, the accumulated ammonia measured seven days after application with trailing hoses was about 67% lower for acidified pig slurry compared to untreated slurry.

• Odour: There have been conducted olfactometric odor measurements for the two trials of acidification of slurry in slaughter houses (Pedersen, 2004 and 2007). The experiments showed no statistically significant effect in terms of odor by acidification. There are examples, that increased odor problems have been discovered locally around the process tank of the acidification unit, which was eliminated by mounting a carbon filter at the process tank. However, JH Smellfighter from JH Agro A/S is a in-house acidification system, configuring with a solid-liquid separation before the slurry is recirculated to the stable, where a considerable smell reduction of 51% in pig stables is achieved according the Danish Technologylist, as well as a lower consumption of sulfuric acid. 

• Methane and other greenhouse gases: Olesen et al. (2018) have estimated the climate effect of in-house acidification at 16 kg CO₂ eq / tonne for cattle slurry and 44 kg CO₂ eq / ton for pig slurry. The assessment includes reduced methane emissions in the barn and storage, extra energy consumption for pumping slurry, reduced nitrous oxide emissions due to a lower need for purchased nitrogen in mineral fertilizers, and indirect reductions in nitrous oxide emissions in the field due to less atmospheric nitrogen deposition. In addition, the effect on nitrification inhibition (mentioned under Effects on water and soil) has an additonal reducing effect on nitrous oxide emissions, whihc according Olesen et al. (2018) is 1.87 tonnes CO_2e per ton of ammonium fertilizer. A laboratory study has shown that emissions of methane from the sulfuric acid treated cattle slurry was 90% lower than the untreated control slurry by measurements over 100 days in a semi-field systems (Petersen and Eriksen, 2008). Another laboratory study showed that emissions of methane from cattle manure stored for seven weeks was 67% lower than the untreated slurry (Hansen, 2008). 

• Laughing gas: The use of acidified slurry has indirect effect on the emission of laughing gas (N₂O) due to its substitution of nitrogen in mineral fertilizers with saved ammonia volatilization in the field fertilizer level (IPCC, 2006). Another indirect effect is seen in case field spreading of acidified slurry with trailing hoses replace injection of slurry (Velthof and Rietra, 2019). There are not assumed any direct effect of slurry acidification on nitrous oxide emissions

• Hydrogene sulfide: There are in several trials seen a substantial lower hydrogene sulfide (H₂S) emission, for instance reported by Riis and Jonassen (2018), who found a hydrogene sulfide emission of 3.8 mg H₂S h^-1 animal^-1 for daily acidified slurry, which is significantly lower than for raw slurry, measure to 9.2 mg H₂S h^-1 animal^-1. Park et al. (2016) found a reduction in sulfur hydrogen emissions of 78.1% from manure that was pH corrected to 5.0, and the Knowledge Center for Pig Production indicates reductions in the level of 67-90% (Jørgensen (2016) and Riis (2016)) for slaughter pig slurry corrected to pH 5.5. Not all studies find reduced emissions of H₂S (Fangueiro et al., 2015), but according Riis and Jonassen (2018) this may be becasue results depends on the measuring point. Some research results are based on laboratory measurements, while others are measured in practice at farms, where a possible degassing of hydrogen sulphide happens during the transfer of slurry between barn and process tank, therefore not inside the barn. 

Effects on water/soil (and management):

Known effects on water and soil deals with atmospheric nitrogen depositions, nitrogen inhibition after field spreading, leaching of excess sulfur, soil acidification, and soil microbiology: 

• Atmospheric nitrogen deposition: Ammonia emissions return in the form of atmospheric nitrogen deposits. Ammonia emissions can be converted by a factor of 0.822 for the content of nitrogen. The deposit point depends on wind directions, and a part may fall down several nhundred kilometers from its source. Before deposition, some is converted to other nitrogenous products, and a smaller portion may be converted to free nitrogen (N₂ gas) and remain in the atmosphere. Atmospheric depositions accounts for approx. 25% of the total load of the Baltic Sea with nitrogen (HELCOM, 2019). The part of the atmospheric depositions than happen on cultivated land could be untilised by agricultural crops, of the fertilising takes the atmospheric depositions into account, whereas the part that is desposited on uncultivatred areas and over the sea inevitalbly would end up, directly or indirectly in the aquatic environment. 

• Nitrification inhibition: Over time, several field trials have been carried out with acidified manure. In some cases, effects are seen on the yields, which cannot be explained by increased nitrogen supply of the crop by the amount of nitrogen that has remained in the slurry due to the acidification. Thus, Nørregaard Hansen (2017) states that SEGES field trials with winter wheat in 2014-2015 have shown yield effects of slurry acidification of 0.17 ton per ha, which is significantly higher than the theoretically calculated effect of 0.07 ton per ha. Other research has explained why yield increases by fertilizing with acidified manure can exceed what is expected. Park et al. (2018) found in an experiment with ryegrass that acidified manure reduced the leaching of nitrate by 17.81%, claiming the reason being the nitrification inhibitory effect of the acidified manure. In line with this, Fangueiro et al. (2016) found that acidification enables increased ammonium content in the slurry without simultaneously increase of nitrification, that acidified slurry is more effective in delaying nitrification than nitrification inhibitors on sandy soils, and that the nitrification inhibiting effect is inversely correlated to the soil buffer capacity. University of Copenhagen (Regueiro et al. 2019) has published results that support this, and found a larger, but not significant, N uptake in plants fertilized with acidified slurry compared to plants fertilized with raw slurry with nitrification inhibitor. It is probably also important that the nitrogen which does not evaporate due to the acidification is in ammonium form, which means that the acidified slurry will have a higher ratio of ammonium to nitrate nitrogen. Unlike nitrate (NO₃^-), ammonium (NH₄⁺) becomes better bonded to the soil particles. Therefore, ammonium stays longer in the root zone than nitrate because it "better" adheres to the soil and is not easily washed out.

• Leaching of excess sulfur: Sulfuric acid contains on a weight basis 32.7% sulfur. The sulfur fertilising need depend on the crop, but would averagely be about 22 kg per ha (Dubgaard and Ståhl, 2018). This means, in relation to the estimated annual need, that a dosis of 30 tonnes of in-house acidified slurry adds enough sulfur fertilizer to the crops for 3.6 years. Sulfur overdosing on the said scale is in line with the annual background deposition in the 60s due to uncleaned flue gases from combustion plants and industry. It could be feared that sulfur overdose would have adverse effects in the aquatic environment. Skwierawska et al. (2008) concluded that a strong overdose with sulfur, ie. an annual allocation of 120 kg of sulfur per hectare after three years showed effects in the form of increased mobilization of phosphorus, which otherwise was present in the soil pool in difficult plant available forms. There were likewise seen changes in concentrations of heavy metals in the soil layers. Leaching of sulfur as a result of severe and yearly repeated overdosing at said level of 120 kg S ha^-1 year^-1 can after a number of years give water a taste of sulfur.

• Soil acidification: It is logical to assume that the use of acidified slurry will lower the soil pH and thus lead to additional need for liming. For this reason, Dubgaard and Ståhl (2018) consider extra liming as a necessity when using slurry acidification, and state that "It is necessary to add extra lime to neutralize the effect of the acid on the fields where the acidified slurry is applied." It is stated that the liming will cost farms with sows and piglets just under € 2,800 a year. With this assertion, reference is made to Olesen et al. (2018), which in Table 14 refers to a theoretical calculation by Peter Sørensen, Aarhus University. Also Foged (2017) and Vestergaard (2015) indicate a need for extra liming, but also without reference to anything other than chemical theory. According Vestergaard (2015), 1.4 kg of agricultural lime is required to neutralize 1 kg of sulfuric acid. However, results from practice do not correspond to the chemical theory: Williams et al. (2020) in six field trials found lower pH values ​​in the test blocks that had been given acidified slurry compared to test blocks without acidified slurry. However, five out of six results were non-significant. During the Baltic Slurry Acidification project, a number of field trials with different crops were carried out during two growing seasons. Peltonen (Undated / 2019) unfortunately does not disclose the number of field trials, but only shows a summary per country. Regarding the pH of the soil before and after the field trials, the results in some cases show that the pH has increased slightly by using acidified slurry, in other cases it is unchanged, in some cases lower and in some cases not measured at all. Another report with detailed results from the field experiments (Kučinskienė et al., 2019) shows the soil pH both before and after for the different trial plots. The report shows that trial plots fertilized with nitrogen commercial fertilizers in some cases have lower pH in the soil after harvest than trial plots fertilized with acidified manure. The field trials conducted in the Baltic Slurry Acidification project have therefore not provided a basis for concluding that acidified slurry reduces the soil pH. In line with this, Fangueiro et al. (2014) in a comprehensive review of the effects of slurry acidification is not mentioning needs for additional liming. The University of Adelaide (undated) states that acidification of the soil is a natural process, which is enhanced by the use of fertilizers, especially nitrogen fertilizers. In Europe, the use of lime for agricultural purposes corresponds to an average of 0.7 kg of lime / kg N supplied (Sutton et al., 2011). The greater uptake of nitrogen in the plants, the less will the soil be acidified - vice versa, the more nitrogen leached, the greater is the acidification of the soil. This corresponds well with the results of pH analyzes of soil samples in the Baltic Slurry Acidification project (Kučinskienė et al., 2019), which occasionally found that artificial fertilized parcels had a lower pH value than parcels fertilized with acidified slurry. Thus, there is no scientific evidence to suggest that fertilization with acidified manure causes greater liming needs than other nitrogen fertilizers.

• Soil microbiology: The microbiology of the soil has a great influence on the many processes that take place in the soil and on the fertility of the soil, the plant availability of nutrients, the degradation of pesticides and chemicals, and the structure of the soil. Microorganisms in the soil are of various kinds and include, for example, bacteria, protozoa, algae and fungi. They make up less than 1% of the earth's plow layer. Marques et al. (2014) found no effect on microorganism activity by fertilizing with acidified slurry. The soil also contains larger organisms such as spring tails, which are 1-2 mm long animals of great importance for the degradation of organic materials in the soil, such as plant residues and animal manure. Annibale et al. (2019) states that with the amounts and acidification used, no decrease in the activity of the spring tails was observed when exposed to treated or acidified slurry, as compared to spring tails in soil without slurry. In fact, in some cases there was a clear positive reaction to the largest supply.

Other effects:

Pedersen (2004) found better production results as effect of using in-house acidification in a trial with 3,683 fatteners:

Conversion efficiency:

Net energy consumption - explanation:

Pedersen (2004) calculated an increased consumption of approx. 3 kWh/m³ slurry by using slurry acidification. The calculation is based on runtime and pump power and is therefore subject to some uncertainty. For the Infarm plant located in Randers (Report 4. Annex B), treating 10,000 m³/y the estimated electricity consumption is 1.8 kWh/m³.

Reagent 1 - explanation:

In the process there will be added concentrated sulfuric acid (H₂SO₄) in amounts as indicated under operational costs, which can differ from farm to farm. Treatments such as nitrification or CO₂ stripping may help in reducing such reagent requirements (possible volatilization must be considered).

Investment cost:

• Investment costs: Dubgaard and Ståhl (2018) state that average capacities / sizes of in-house acidification plants for sow, fattener and cattle farms are 9,700, 12,400 and 5,100 tonnes per year respectively, and that the corresponding plant investment typically is € 184,000 for pig farms and € 89,000 for cattle farms. In practice, the size of the investment will vary from farm to farm, depending on conditions such as the interior of the barn as well as possibilities for placement of process tanks and storage tanks. A depreciation over 15 years is assumed.

• Savings on solid cover on slurry tanks: Some countries require slurry stores to have solid covers in some cases. This is for instance the situation in Denmark, where slurry tanks must have solid cover if they are situated within 300 metres from sensitive nature. However, for Denmark this requirement is equalised with the use of acidified slurry, due to the substantial reduced emissions. The price of solid covers on slurry storage tanks depends a lot on the diameter of the storage tank. A solid cover on a slurry storage tank could in many cases make it 100,000 € more expensive.

Operational costs - explanation:

• Consumption of sulfuric acid: The exact consumption of sulfuric acid is determined by titration, and varies with slurry qualities, probably depending in the chemical composition of the slurry apart from the animal type. However, some general indication for the consumption can be given. According Dubgaard and Ståhl (2018), the consumption of sulfuric acid is 9.6, 8.7 and 5.7 kg per tonne of manure for sow, piglet and cattle slurry respectively. The following table is based on information from Hansen & Knudsen (2017). Current prices for sulfuric acid is about 0.15€ per kg.

  	Consumption of concentrated sulfuric for in-house acidification, liter / kg
  Cattle slurry	4.5 / 8.3
  Pig slurry	3.5 / 6.4

• Costs of service scheme: Subscription to a service schemes costs € 2,000 per year for pig farms and € 1,350 per year for cattle farms (Personal information, Kurt West, JH Agro A/S, June 2020). The service scheme includes an annual inspection where the plant is checked and wear parts replaced.






 

Quantifiable income - text:

• Avoided health care sector costs due to reduced ammonia emission: Ammonia is one of the most important factors in air quality affecting our health in terms of number of sick days and hospitalisation, prevalence of asthma, bronchitis and lung cancer as well as other respiratory disorders, and in relation to life expectancy and premature births and abortions. The societal value of the quality of the air is therefore important. Sutton et al. (2011) have estimated the health loss of 1 kg of nitrogen in the form of ammonia evaporation to, for example  10 € per kg for Denmark, 27 € per kg for the Netherlands, and only 2 € per kg for Lithuania, where the air quity in general is better than the other mentiond countries. 

• Value of reduced CO₂ emissions: The traded value of CO₂ emission reductions are seen from https://markets.businessinsider.com/commodities/co2-european-emission-allowances. 

• Uncertain effect on crop yields: In the Baltic Slurry Acidification project (https://www.balticslurry.eu), there were on basis of field trials in 2017 and 2018, with a number of crops in seven countries, no basis for concluding any statistically significant effect of fertilizing with acidified manure. Some field trials (Kučinskienė et al., 2019) yielded higher and others lower, and yield results were likely influenced by atypical weather conditions with extreme rainfall in 2017 and extreme drought in 2018. SEGES has generally shown a yield effect in national experiments with acidified manure (Nørregaard Hansen, 2017), a large part of which can, however, be attributed to the extra nitrogen supply caused by acidified manure, and the experiments were carried out during a period where the fertilizer norms were determined by political decision, regulated to a level below the economically optimal.

  Williams et al. (2020) informs of significantly higher yields in acidified manure parcels under English conditions, but it is not stated whether the additional yields exceed the expected relative to the extra nitrogen allocation.

  In order to assess yield effects, the fertilizer level is crucial in relation to the N-fertilizer response curve. If the level is at the productivity optimum, where the response curve is flat, no yield effect can be expected as a result of the crop being supplied with a higher amount of plant nutrients, primarily nitrogen. A relatively small yield effect can be expected from fertilization at economically optimal fertilizer level, and a larger yield effect when fertilizing below the economically optimal level. Unfortunately, it is generally not stated what the yield effect could theoretically be expected or how the fertilizer level was in relation to the response curve when results of acidified manure fertilizer trials are presented.

  

  Figure 2: In order to be able to interpret the yield effects of field trials with barnyard manure, it is crucial whether the fertilizer level corresponds to productivity optimum (A), economic optimum (B), or below economic optimum (C).

• Reduced needs for purchase of nitrogen in mineral fertiliser: Acidification of manure means that the content of nitrogen in the manure at storage is 7-13% higher than in normal manure handling. By application with trailing hoses of acidified slurry, a 20-25% increase in fertilizer effect (bio-availability) is expected (Kai et al., 2008), while the nitrogen effect of acidified slurry is not increased when comparing to injection (Sørensen and Eriksen, 2009).All in all, according Nørrgaard Hansen (2017), there are roughly captured 1 kg N per ton slurry due to avoided ammonia emission, which can be saved in purchase of nitrogen in mineral fertiliser. A typical price for nitrogen in mineral fertiliser is 0.9-1 € per kg. 

• Reduced needs for purchase of sulfur in mineral fertiliser: According the above concerning the consumption of sulfuric acid, a crop that for instance is given 30 ton in-house acidificed slurry with 6.4 kg sulfuric acid per ton with a sulfur content of 32.7% would thus receive 63 kg sulfur. This is for most crops more than they need - an average need would be in the level of 22 kg S per ha, while for instance rape seed could have needs up to more than 50 kg S per ha. Therefore, a farm that use in-house acidified slurry for fertilising would be able to save its entire need for purchase of sulfur mineraler fertilsier. A price indication for sulfur in mineral fertiliser is about 0.4 € per kg.
  

• No needs for starter fertiliser for maize: When corn is grown, the perception is that i) Sufficient amounts of plant-available phosphorus are important for starting the growth, and ii) this should be ensured by the supply of high water solubility phosphorus in the form of commercial fertilizers, even if the maize field is fertilized with slurry, the phosphorus content of which is of no value in this regard. It is recommended to give the corn 8-15 kg of water soluble phosphorus in addition to the basic fertilizer. Regueiro et al. (2019) states that phosphorus in slurry has a water solubility of up to 70% by being acidified to ph 5.5, which is a water solubility that is three times higher than that of phosphorus in raw slurry. Higher water solubility of phosphorus is also obtained by acidification of solid separation fractions and digestates. If 30 tonnes of in-house acidified slurry are fertilized per hectare, between 15 and 19 kg of water-soluble phosphorus per hectare will be supplied, cf. above is more than the recommended amount. Water soluble phosphorus has a market price of about 1.4 € per kg. 

• Savings on slurry injection: Some countries has requirements to injection of slurry in some cases. This is for instance the situation in Denmark, where slurry must be injected on bare soils without crop, and on grass fields. However, for Denmark this requirement is equalised with the use of acidified slurry which in these cases can be spread with band-laying systems. Injection has due to a smaller working width and larger energy consumption an extra field spreading costs of app. 0.5 € per ton (Foged, 2017). 

• Savings on floating cover on slurry tanks: Some countries require slurry stores to have floating covers on slurry storage tanks. This is for instance always the situation in Denmark, unless for slurry storage tanks that muts have solid cover. According Dubgaard and Ståhl (2018), it costs annually in the level of 2,000 - 2,700 € for a pig farm to re-establish a floating layer after the slurry tank is emptied, whereas a floating layer is formed by itself in tanks for cattle slurry.