Value Added Products from Pyrolysis of Sludges from Pork Production

Value Added Products from Pyrolysis of Sludges from Pork Production

Corresponding author: Dr. Lawrence M. Pratt, Department of Chemistry and Environmental Science, Medgar Evers College, The City University of New York, 1638 Bedford Ave, Brooklyn, NY 11225, Email:lpratt@mec.cuny.edu

Abstract

Treatment of waste products from pork production results in two types of sludge. The first is high-fat sludge from physical treatment and dewatering. That sludge is further treated by a proprietary biological process, resulting in a second type of sludge that generates less odor. Pyrolysis of both sludges yields an oily distillate, an aqueous distillate, and biochar. The oily distillate from the physical treatment consists primarily of aliphatic and aromatic hydrocarbons that are components of diesel fuel, and thus, has commercial potential as a fuel precursor. The oil from the biological treatment is a complex mixture with less commercial potential. The biochar from both types of sludge was evaluated for its potential as a fuel and as a soil amendment.

Keywords: Pork sludge; Biofuel; Hydrocarbons; Biochar; Aromatics

Introduction

Pork production results in waste sludge from the inedible por- tions of the swine. These include blood, skin, and residue of pigs, fat, and hair, as well as residual water. Slaughterhouse wastewater typically contains fats, proteins, cellulose, and water soluble organic compounds [1]. The sludge initially un- dergoes a physical treatment involving the unit operations of adsorbtion, coagulation, floatation, and disinfection. The re- sulting sludge with variable water content is sent to a hold- ing area. A typical pork plant in Vietnam produces about 20 cubic meters of this sludge per day. It is then further treated by a proprietary biological process, in which the volume and odor are greatly reduced. The high fats, oils, and greases (FOG) content of pork waste makes it of interest for production of biofuels. Several other methods for slaughterhouse waste treatment have been developed, including oxidation and an- aerobic digestion [2-6]. We have had success in obtaining ker- osene-like hydrocarbon mixtures from other sources of waste oil, particularly brown grease.

Brown grease, a fatty material removed from sewer lines and the sewage treatment process, has been shown to produce a kerosene-like mixture of aliphatic hydrocarbons when heated in limited air to temperatures of up to about 400 oC [7, 8]. Work currently in progress shows that oil extracted from municipal garbage undergoes similar pyrolysis reactions, which has led to a search of other oily waste products that have potential for fuel production via pyrolysis. Sludge from pork production is one such material. The high temperatures reached in this pro-

cess also destroy pathogens, many of which require tempera- tures in excess of 100 oC for destruction [9].

The primary byproduct of this process is biochar, a carbon-rich material with many useful properties. Biochar with sufficient- ly high carbon content is used as a fuel, e. g. charcoal. It has been used to remove metal [10] and organic [11] contaminants from waste water. It is used extensively as a soil amendment, as it affects the water retention, nutrient uptake, and crop yields [12, 13]. Although biochar contains polycyclic aromatic hydrocarbons (PAH’s), a recent study found no significant in- crease in free PAH’s in biochar amended soil [14].

Materials and Methods

Sludge from physical and biological treatment of pork produc- tion waste was obtained from the Vissan plant in Ho Chi Minh City, Vietnam. Samples of both types of sludge were filtered to remove the water and air dried prior to pyrolysis. A reactive distillation setup consisting of a round bottom flask, a Dean- Stark trap, and a reflux condenser was used for the pyrolysis, with the aqueous distillate forming the lower layer in the trap. The heating mantle was initially set at heat setting 4 out of 10, and ramped up one unit every 15 minutes to setting 7, resulting in a flask temperature of approximately 340 oC. Heating con- tinued until no more aqueous or oily distillate formed, which required approximately 8 hours. After pyrolysis the aqueous layer, oil layer, and biochar were collected and the mass of each determined. The aqueous layer was extracted twice with 10 mL portions of ethyl acetate, and the composition of the organ- ic material in both fractions was determined by GC-MS with an Agilent 7890 chromatograph using a MS 5973N detector and capillary column DB-5 MS 30 m long x 0.25 mm id x 0.25 μm active phase film thickness. A temperature profile of 50 oC for 3 minutes, ramp of 12 oC/minute to a final column temperature of 280 oC for 30 minutes was used.

Analysis of the biochar for nitrogen was performed by the Kjeldhan method. Phosphorus and potassium were analyzed by ICP-MS (Agilent model 7900) after digestion with a H2SO4, HF, HNO3 mixture. To ensure compliance with EPA rule 503 for biosolids, heavy metals were analyzed by ICP with a Shimadzu model ICPE-9000 machine. Samples were prepared as follows. All glassware and sample vessels used were first washed with EDTA to render them metal free. A 15 mg sample was placed in a PTFE vessel and then suspended in mixture of concen- trated H2SO4 (5 mL), concentrated HNO3 (1 mL), and concen- trated HCl (2 mL). The samples were digested on a Milestone Ethos EX Microwave Extraction System. The samples were first heated to 200°C over 10 minutes, then heated to 220°C over five minutes, and maintained at 220°C for 20 minutes. After extraction, the samples were diluted with 30 mL of DI water in an ice bath. Solid NaOH was added to each sample until the acid was neutralized (pH ~7). 1 mL of each sample was placed in a 25 mL volumetric flask which was then filled with DI water

to the mark. The samples were analyzed on a Shimadzu ICPE9000 atomic emission spectrometer.

Ash tests were performed were performed on each biochar sample to determine the amount of inorganic material. Sam- ples of each biochar (approximately 0.5 g) were weighed in a crucible, and heated to burn off the organic material until con- stant mass. The ash was reported as a percentage of the orig- inal mass.

Results

Product yields and composition

The yields of pyrolysis products are shown in Table 1. The sludge from the physical treatment generated oil in nearly 10% yield, and over 50% yield of biochar, both based on the dry mass of the sludge. The biochar from the physical treatment retained a significant amount of oil, and washing of the char with dichloromethane recovered 30.9% of the biochar mass as additional oil, as described below. The result was a combined oil yield of more than 25%. Significantly lower yields of oil and slightly lower yields of biochar were obtained from the sludge from the biological treatment.

Opponents of developing renewable sources of petroleum products have argued that the cost of renewable is too high and supply does not meet the demand. While the supply of each individual renewable source may not meet the demand, numerous sources taken together can have a significant effect on the supply. Petroleum producers are currently developing “alternative” sources of fossil fuels, such as oil shale and tar sands, often with major impacts on the environment. The oil derived from the physical treatment sludge is comparable to the hydrocarbon generating potential of oil shale derived kero- gen, based on mg HC/g rock [15]. The technology described in this paper will benefit the environment, and will likely be ap- plicable to other sources of oily and greasy waste in the future.

Sludge

type

Initial mass (g) Aqueous

distillate (g)

Oil (g) Oil %

yield

Char

(g)

Char %

yield

Physical 60.66 8.27 6.07 10.0 31.96 52.7
Physical 84.52 13.25 9.43 11.2 44.41 52.5
Physical 71.40 13.14 6.13 8.58 40.66 56.9
Biological 65.15 20.82 4.66 7.15 30.60 47.0
Biological 47.60 14.90 2.75 5.78 24.32 51.1
Biological 34.93 10.25 2.15 6.15 17.13 49.0

Table 1. Pyrolysis products of pork sludge.

The compositions of oil distillate for 3 samples each of the sludges are shown in Table 2. The percent compositions of

the products were obtained by peak integration. The identity of the peaks were determined from the history of the sample, known compounds, and mass spectral analysis. The physical- ly treated sludge gave the highest hydrocarbon content, with the total of aliphatic and aromatic hydrocarbons ranging from 84 to 94% of the total oil by mass. The aliphatic hydrocar- bons were primarily C7 to C18 alkanes and the corresponding 1-alkenes, with smaller amounts of internal alkenes and high- er alkanes. The remainder was nitrogen compounds, which consisted primarily of nitrogen heterocycles and nitriles, and other compounds, which included small amounts of alcohols, ketones, and other compounds that could not be readily identi- fied. The sample to sample variation can be attributed, in part, to the non-homogenous nature of the sludge.

The biologically treated sludges produced high aromatic con- tents of the pyrolysis oil, which was primarily toluene, but also included small amounts of ethylbenzene, styrene, and other substituted benzenes. The alkane and alkene content was low- er, between 30 and 37% by dry mass, and the nitrogen com- pounds were higher in the biological sludge distillate. There was more sample to sample variation, compared to the physi- cally treated sludge.

The physically treated sludge is a promising feedstock for green diesel due to the high hydrocarbon content. Diesel fuel can contain up to 35% aromatic compounds, and catalytic hy- drogenation of the alkenes to the corresponding alkane will re- sult in a cleaner burning fuel. The sludge treated by the propri- etary biological processhas less potential as a fuel, but the high aromatic content, particularly of toluene, makes it a potential source of petrochemicals, particularly toluene and benzoic acid. The latter is derived from toluene and other monoalkyl- benzenes by oxidation.

Sludge type Aromatic

hydrocarbons

Alkanes

and alkenes

Phenols Nitrogen

compounds

Other
Physical 13.0 81.5 0 2.9 2.6
Physical 10.7 78.4 0 6.5 3.6
Physical 10.4 73.7 0 10.4 5.4
Biological 25.2 30.8 16.1 20.0 3.6
Biological 38.2 36.5 0 17.7 8.3
Biological 33.0 31.7 1.0 12.8 21.6

Table 2. Percent composition of oil distillate from pork sludge by GCMS analysis.

The aqueous distillate from the physically treated sludge (8.27

g) was extracted twice with 10 mL ethyl acetate, and the sol- vent was evaporated to obtain 0.38 g of residue. This minor organic component contained palmitic acid, hexadecanitrile,

and some pyrroles. Because of the miniscule quantity of or- ganics, the aqueous fraction is not likely to be of commercial importance. Similarly, extraction and evaporation of the aque- ous distillate from the biologically treated sludge gave 0.69 g of residue consisting largely of nitrogen heterocycles. These quantities are not likely to be commercially viable after ex- traction and separation of the components. Any commercial value of the wastewater from this process may come from the nutrients for plant, microbe, or algae growth, which would also serve to remediate the waste stream [16, 17].

Biochar characterization

To test the potential of the biochar for fuel, samples of the char resulting from the pyrolysis of the two sludges were ignited by an alcohol lamp and the burning was observed. The physical- ly treated sludge biochar ignited easily and continued to burn after removal from the alcohol flame. The biologically treated sludge biochar sample was difficult to ignite, and did not con- tinue burning after removal from the flame. Further examina- tion of the biochar from the physically treated sludge showed it to be quite oily, and when the oil was removed by washing with dichloromethane, the remaining char was also difficult to ignite, thus limiting its potential value as a fuel.

Biochar is, however, useful as a soil amendment, and the ni- trogen, phosphorus, and potassium contents of the unwashed biochars were determined. The Kjeldhan method for nitrogen analysis was used, and the physically and biologically treated sludge biochars had nitrogen contents of 4.13 and 5.74%, re- spectively. ICP-MS was used for phosphorus and potassium analysis. The physically treated char measured 27.88 mg/g P and 1.34 mg/g K, while the biologically treated char measured 58.31 mg/g P and 2.30 mg/g K.

Heavy metal contamination of soil is a concern for agricultur- al use, and the United States EPA has placed a ceiling on the elements As, Cd, Cu, Pb, Hg, Mo, Ni, Se, and Zn. The measured values of these elements in the two biochars (dicholormethane washed and unwashed), and the EPA ceiling values, are shown in Table 3. Values of ND indicate that the element is below the limits of detection by this method. Washed samples removed some of the organic material in the form of oil while leaving behind the inorganic material, thus resulting in higher metal content in some of the washed samples. From this data we conclude that all 4 samples of biochar comply with EPA rule 503 for land application of biosolids in agriculture.

The inorganic content of the biochar was determined from ash tests, and the results are shown in Table 4. The increased ash content of the char from the physically treated sample after washing is consistent with the high percentage of oily organic material remaining in the char after pyrolysis. In contrast, the percent ash in the biologically treated sample remained nearly unchanged after washing, which is consistent with the lower

oil content. The higher ash content of the biological treatment is consistent with the loss of organic material. Although this particular treatment process is proprietary, biological treat- ment generally includes aerobic and/or anaerobic digestion of the organic material [18].

Element Physical

(unwashed)

Physical

(washed)

Biological

(unwashed)

Biological

(washed)

EPA

ceiling

Detection

Limit

As ND ND ND ND 75 4.1
Cd ND ND ND ND 85 4.1
Cu 364 531 981 1171 4300 4.1
Hg ND ND ND ND 57 41.3
Mo ND ND ND ND 75 41.3
Ni ND ND ND ND 420 4.1
Pb ND ND ND ND 840 41.3
Se ND ND ND ND 100 41.3
Zn 59 30 ND ND 7500 4.1

Table 3. ICP measured concentrations (mg element/kg biochar) of elements regulated by EPA rule 503. ND represents an element that was not detected, i.e. below the detection limit

Unless the biogas (CO2 and CH4) is collected from the anaero- bic digestion, it will escape into the atmosphere as greenhouse gases. Thus, interception and use of the organic material after the physical treatment process helps to reduce the contribu- tion of this industry to global warming.

Biochar from sludge

type

Unwashed Washed

(dichloromethane)

Physical treatment 24.7 38.3
Biological treatment 43.8 42.7

Table 4. Percent ash in biochar samples.

Pork sludge poses an environmental problem for disposal, but we have shown its potential for use as a fuel, chemical, and bio- char precursor. Although by itself it would be only a minor source of fuel oil, the oil obtained is chemically similar to that obtained from brown grease and oil extracted from municipal solid waste. Thus, we anticipate that these and other similar sources of waste oil can be combined and used in a single re- finery. The combined profits from the oil, biochar, and savings in sludge disposal costs will enhance the economic viability of this process.

Conclusion

Two types of pork sludge were evaluated for fuel and biochar production. The physically treated sludge generated the highest oil yield, which consisted primarily of aliphatic and aro- matic hydrocarbons. The aliphatic component is chemically similar to the product obtained from brown grease. The bi- ologically treated sludge is a potential source of commodity chemicals. Biochar from both sludges meet the EPA standards for heavy metals for land applications.

Acknowledgements

This work was supported in part by NSF grant #CBET 1507069.

Thanks to Vissan for providing samples of sludge.

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