slovak society of chemical engineering institute of...

Slovak Society of Chemical EngineeringInstitute of Chemical and Environmental Engineering

Slovak University of Technology in Bratislava

PROCEEDINGS37th International Conference of Slovak Society of Chemical Engineering

Hotel Hutník

Tatranské Matliare, Slovakia

May 24 – 28, 2010

Editor: J. Markoš

ISBN 978-80-227-3290-1

Bodík, I., Sedlácek, S., Kubaská, M., Hutnan, M.: Perspectives of biogas production from restaurant wasteson slovak municipal wastewater treatment plants, Editor: Markoš, J., In Proceedings of the 37th Interna-tional Conference of Slovak Society of Chemical Engineering, Tatranské Matliare, Slovakia, 1119–1125,2010.

Perspectives of Biogas Production from Restaurant Waste on Slovak Municipal Wastewater Treatment Plants

I. BODÍK*, S. SEDLÁČEK, M.KUBASKÁ, M.HUTŇAN

Institute of Chemical and Environmental Engineering, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovak Republic

(*E-mail: [email protected]) Keywords anaerobic processes, biogas, biodegradable waste, fermentation, wastewater treatment,.

The presented contribution deals with the possibility of usage of food and restaurant wastes to the increase of the biogas production in the Slovak municipal WWTPs. In 45 Slovak WWTPs (from the 520 total) the process of anaerobic fermentation with the biogas production is operated. The total volume of fermentation tanks is about 162 000 m3 but the total daily biogas production is only app. 51 500 m3/day. The contribution describes the several-month laboratory research of the utilisation of food wastes as the organic source of biogas production. The results have confirmed the high potential of the biogas production from food wastes – in average about 1000 L of biogas to 1 kg Xorg. The influence of sludge water from food wastes to biological process of treatment was also evaluated.

INTRODUCTION

The biogas production in Slovakia is dominant mostly on municipal WWTPs. Recently some agricultural biogas stations were constructed and few others are considered to be constructed but according to misty energy policy of the government in the area of renewable energy sources they represent only small share of biogas production. The energy policy of the state leads to important backward to the developed EU countries and Slovakia produces very small amounts of biogas energy. As it is known from [1], the total capacity of 45 examined Slovak WWTPs is more than 5.5 Mil PE. High amount of WWTPs has specific volumes of digestion tanks extremely high (higher than 80 – 100 L/PE) what in most cases can assign non-effective operation of sludge management (in some WWTPs part of volumes is out of operation, but into the statistical data total volume was reported). Average retention time of sludge in digestion tanks in Slovakia (together with storage tank) is about 33 days and is ranging between 12 to 92 days. All presented data indicate that most of WWTPs has over-dimensioned or insufficiently charged digesters and have free capacities for treatment of external substrates to biogas production. In 2007 in Slovak WWTPs about 51 500 m3 of biogas was produced daily that represent annual production of almost 18 mil m3 of biogas. The average value of specific biogas production (litre of biogas/PE) in Slovak WWTPs represents 20 L/PE. Yearly production of 18 mil m3 of biogas theoretically represents about 113 GWh of energy. Only in 18 from 45 executed WWTPs have installed the equipment for electrical power production (co-generation unit also combined heat and power, CHP) with total installed performance of 3.3 MW (individual WWTPs with performance range of 35 – 800 kW). In the surveyed WWTPs the 35 000 kWh of electrical power was daily produced what represents 12.7 GWh annually. According to the information from water companies many WWTPs have serious interest of installation or enlargement of production of electrical power on CHP systems [1].

37th International Conference of SSCHEMay 24–28, 2010, Tatranske Matliare, Slovakia Le-Th-2, 060p.pdf

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Possibilities of increase of biogas production and its effective utilisation The most of Slovak municipal WWTPs has free technical and technological capacities for biogas production increase. It is clear that increase of biogas production from sludge produced in WWTP is limited (only different forms of sludge decomposition) and only external sources of organic materials can be considered. The range of used external sources of organic materials is wide and most used materials are: - Agricultural products (harvest residua, special grown plants etc.); - Food intermediate products (waste, inconvenient raw materials, food with low quality etc); - Industrial intermediate products and wastes (chemical industry, organic materials, etc.); - Wastes from restaurants and markets, food after warranted time, green municipal waste; - Separately collected organic wastes from inhabitants; - Wastes from animal husbandry, slaughter-house, perished animals, etc.; - Other sources. The use of external organic energy substrates could (in most cases) complicate the technology process and it would be necessary to add some technological units (sanitary and pasteurizing reactor) but the resulting effect would be in all cases definitely positive – important increase of biogas production and from long-term view improvement of economical indicators of the operation. Restaurant (food) waste as substrate for anaerobic digestion Food waste is a growing issue, and the disposal of it is controversial, given increased food prices and the resources required. Food waste makes up an estimated 8.4% by weight of municipal solid waste in the United States, and on average in the UK, each household buys 16.5 kg of food a week and throws a third of it away. The food waste includes uneaten food and food preparation leftovers from residences, commercial establishments such as restaurants, institutional sources like school cafeteria, and industrial sources like factory lunchrooms. Food waste is the single-largest component of the waste stream by weight in the USA – Americans throw away about 43.6 million tons of food each year, which represents 160 kg/cap.year [2]. The recent papers reporting on the status of organic waste recycling in the EU suggested that, with the exception of Greece, large parts of Spain, Portugal, France and Ireland, all the old European countries, had started to collect organic waste separately to recycle it. Treatment of food waste by Anaerobic Digestion (AD) was also reported – with an estimated total capacity of 3.5 million tonnes. [3]. Cho and Park [4] determined the methane yields of different food wastes at 37 °C and 28 days of digestion time. They were 482, 294, 277, and 472 mL/gVSS for cooked meat, boiled rice, fresh cabbage and mixed food wastes, respectively, which correspond to 82%, 72%, 73% and 86% of the stoichiometric methane yield, respectively, based on elemental composition of raw materials. Heo et al. [5] evaluated the biodegradability of a traditional Korean food consisted of boiled rice (10–15%), vegetables (65–70%), and meat and eggs (15–20%) and reported that after 40 days a methane yield of 489mL/gVSS could be obtained at 35 °C. Zhang et al [6] characterized potential of food waste as a feedstock for anaerobic digestion. The daily and weekly variations of food waste composition over a two-month period were measured. The anaerobic digestibility and biogas and methane yields of the food waste were evaluated using batch anaerobic digestion tests performed at 50 °C. The daily average moisture content (MC) and the ratio of volatile solids to total solids (VSS/TSS) determined from a week-long sampling were 70% and 83%, respectively, while the weekly average MC and VSS/TSS were 74% and 87%, respectively. The nutrient content analysis showed that the food waste contained well balanced nutrients for anaerobic microorganisms. The methane yield was determined to be 348 and 435mL/gVSS, respectively, after 10 and 28 days of digestion. The average methane content of biogas was 73%. The average VSS destruction was 81% at the end of the 28-day digestion test. The results of this study indicate that the food waste is a highly desirable substrate for anaerobic digesters with regards to its high biodegradability and methane yield.


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The objectives of this study were to describe the food waste digestion in the laboratory conditions with the aim of characterization of the basic technological parameters such a specific biogas production, methane content in biogas, optimal organic load etc. The next aim of study was to define the influence of food waste doses on quality of sludge water on municipal WWTP, especially, on nitrogen and phosphorus concentration, COD, pH, lower fatty acids, etc.

MATERIALS AND METHODS

Description of laboratory equipment The laboratory modeling of the biogas production from food and restaurant wastes was realised in the anaerobic reactor (producer ASIO Bytča, Slovakia) with the total volume 13 L. The reactor was warmed up to 37 – 38 °C by thermostat TS – 050 SRCI (producer MERLIN GERIN) and stirred by IKA RW 14basic. The evolve speed was 100 in minute, the stirring started every thirty minutes and the duration of stirring was 15minutes. The biogas was carried to biogas flow meter where the production of biogas was observed.

Photo 1. Laboratory model of anaerobic digestion

The laboratory model was filled with the anaerobic sludge from WWTP Devínska Nová Ves (Slovakia). The initial values of the sludge were: total suspended solids (TSS) 13.7 g/L, volatile suspended solids (VSS) 54 % and pH 7.18. The dosing of the substrate was starting on November 5th, 2008 and stopped May 18th, 2009. The restaurant (food) wastes from the canteen of Faculty of Chemical and Food Technology of the Slovak University of Technology Bratislava were used as the single substrate. The operator of the canteen is the company Slovgast that prepares lunches for 600-800 persons daily. Generally the menu includes six main daily meals that ensure sufficient diversion of the substrate. Every new substrate was mixed, hygienised at 70°C during one hour and the values of pH, chemical organic demand, ammonia nitrogen, total nitrogen and phosphorus, total suspended solids and volatile suspended solids were measures.


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Basic composition parameters of tested food waste The range and average substrate composition is introduced in Tab 1. The range in some measured parameters as COD, Ptot, TSS, VSS and pH is relatively wide. This variability depends on type of meals, portion of moistures foods (soups, sauces etc.). The nutrient contents, balances between COD : Ntot : Ptot showed that this tested food waste represents well balanced feedstock for anaerobic digestion with expected high biological degradation in anaerobic condition. The food waste had very high portion of organic (volatile) solids comparable with other studies [5,6].

Table 1. The basic technological parameters of food waste.

RESULTS AND DISCUSSION

The laboratory reactor began its operation on November 5th, 2008 (first day) by addition of the first substrate dose. The initial value of the volumetric reactor load was Bv = 0.41 kg VS/ m3.day, the load was gradually being increased to the value of 3 kg VSS/m3.day.

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Figure 1. Development of volumetric load and specific biogas production during the test.

SUBSTRATE RANGE AVERAGE COD (mg/L) 150000 - 320000 243675

NH4-N (mg/L) 645 - 896 761 Ntot (mg/L) 3000 - 3900 3342 Ptot (mg/L) 300 - 808 501 TSS (g/L) 85 - 282 130 VSS (g/L) 77 - 270 121 VS/TS (%) 91 - 95 92

pH 4.82 – 8.72 5.3


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From the start of the process the biogas production was monitored daily. The volume of the produced biogas was increased gradually depending on the increasing reactor load. The highest production of the biogas 55.4 L/day was reached in 147th day at reactor load 3 kg VSS/m3.day. The average specific biogas production from food wastes was around 1000 L of biogas from 1 kg VSS of waste during the whole duration of the measurements. The biogas analyses were realised on the gas analyser GA1125 (producer Geotechnical Instruments) and the average methane concentration in biogas was 52.5 %.

In the present the food (restaurant) wastes as the organic substrate for biogas production are not used in Slovakia, but the potential of their utilisation is very high. The great amount of existing fermentation tanks is not completely exploited and the addition of even small amount of food waste can effect positively the biogas production. Theoretical risk of the solution can be the presence of nitrogen in this type of wastes that does not affect directly the process of biogas production, but it can represent the important complication in the waste water treatment process in WWTP.

For the purpose of the determination of the influence of nitrogen from food wastes to the process of wastewater treatment the program in MS Excel 2007 was elaborated that is able to calculate the increase of individual pollution values (COD, BOD5, Ntot, Ptot) in the inflow to WWTP after the entry of basic technological parameters. The output of the program is the definition of sludge water quality depending on food waste dose volume. The addition of food wastes to WWTP with the dimension of about 100 000 PE could not cause important influence of the treatment process. If the volume of the added substrate was up to 25 m3/day, the change in the biogas production or waste water treatment processes would be negligible (Ntot increasing of 1,5 mg/l). At the dose 50 m3/day of food wastes the increase of the nitrogen concentration in inflow to WWTP represents about 2.9 mg/L. The same dose of food waste into smaller WWTP (50 000 PE) can cause a serious problems with nitrogen removal (Ntot increasing – 5,9 mg/l) which should be eliminated with upgrading of denitrification process on WWTP.


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Figure 2. Influence of food waste doses on increasing of Ntot concentration in influent of WWTP through sludge waste.

CONCLUSIONS

As resulting from the up-to-now experiments held during several months in Faculty of Chemical and Food Technology of the Slovak University of Technology Bratislava it can be stated that utilisation of food wastes as the substrate for biogas production increase is possible and very effective. However it is necessary to dose the substrate to WWTPs with sensitivity, to define the composition of the wastes, to define optimal load of fermentation reactors. The next important condition for the utilisation of food wastes is the definition of their influence to the quality of input parameters of waste water namely from the point of view of nitrogen and its removal from waters.

Acknowledgement: Authors want to express thanks to Slovak Grant Agency APVV 0019/09 for providing financial support. The presented contribution was created also as a part of project SK00023 financed by Norwegian Financial Mechanism, Financial Mechanism of EEA and the State Budget of the Slovak Republic (www.eeagrants.com).

REFERENCES

1. Bodík I., Hutňan M., Sedláček S., Kubaská M.: Produkcia bioplynu na komunálnych čistiarňach odpadových vôd na Slovensku. Vodní hospodářství 59/11 – príloha Čistírenské listy 6/2009 pp.I-III (2009) (in Slovak).


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2. US EPA: Waste not, want not: feeding the Hungary and reducing solid waste through food recovery. EPA report 530-R-99-040 (2002).

3. Kidby D.: Food, glorious food. On-line journal Waste Management World, www.waste-management-world.com (2009).

4. Cho, J.K., Park, S.C.: Biochemical methane potential and solid state anaerobic digestion of Korean food wastes. Bioresour. Technol. 52 (3), 245–253 (1995).

5. Heo, N.H., Park, S.C., Kang, H., 2004. Effects of mixture ratio and hydraulic retention time on single-stage anaerobic co-digestion of food waste and waste activated sludge. J. Environ. Sci. Health A39 (7), 1739–1756 (2004)

6. Zhang R., El-Mashad H.M., Hartman K., Wang F., Liu G., Choate Ch., Gamble P., Characterization of food waste as feedstock for anaerobic digestion. Bioresource Technology 98, 929-925 (2007).


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