Integrated Bio-Systems in Zero Emissions Applications Proceedings of the Internet Conference on Integrated Bio-Systems   Editors: Eng-Leong Foo & Tarcisio Della Senta. 1998 http://www.ias.unu.edu/proceedings/icibs

ABSTRACT

In Italy, recently, at the end of the eighties, a new generation of biogas systems for animal (mainly pig) slurry were developed which are extremely simplified and low-cost, involving the use of plastic cover over a slurry storage tank. These systems have been developed not only for the purpose of energy recovery but also for controlling odours and stabilizing the wastes. Though no official census has been made, information gathered from the firms that produce this type of system indicates that approximately fifty of these plants have been installed in Italy up to now. The systems operate at low temperature or at a controlled temperature. Also part of this scenario is a provision of the Italian government of 1992 that offers incentives for self-production of electric energy from biomasses; this could translate into renewed interest in biogas systems for pig breeding. However, this rule is today under revision due to public budget restriction. CRPA had monitored some of these biogas plants, in particular a plant installed in a large pig farm located in the province of Parma. Here the biogas system was created by adapting two tanks, of the same size, used for slurry storage. The total useful volume is about 600 m³. The biogas is used for a co-generator that can supply about 50 kW of electric power and 120 kW of thermal power. The productive parameters, energetic balance and economical analysis of the biogas plant are presented in the paper: the biogas production is about 118,000 m³/year, about 357 m³/t lw · year; the production of electric energy is about 191,000 kWh/year and the pay back time is 4.5 years. Moreover CRPA and Agricultural Engineering Istitute of Milano University carried out a market analysis about cogenerators interesting for pig breeding. The results of this study are reported in the paper.
 

1. INTRODUCTION
In Italy the diffusion of Anaerobic Digestion plants for farm and agro-industrial wastes started at the beginning of the eighties and lasted about ten years. During that period, more than hundred farm biogas plants and about twenty five large agro-industrial plants were built. A survey carried out by ENEA in 1983 showed that over 60 farm manure anaerobic digesters were in operation and more than 20 were under construction at that date (Tilche et al., 1983). The growth lasted only few more years, during which some public funds for anaerobic digestion were still available. Most of farm plants were treating pig wastes, that in Italy represents an "industrialised" animal farming, carried out in large and very large units without land, while most of agro-industrial plants were treating distillery effluents. Also some centralised projects for digesters treating wastes of many different farms started during these years.
Since then the situation has changed substantially, particularly because many of the systems constructed at that time are no longer in operation. The causes can be found in the motivations that led to the installation of the initial systems. In reality, energy saving was only one reason, and not the main one, for farmers to build a digester. The "hypothetical" treatment benefit offered by the technology was often the most important reason, because "industrial" farms had to treat their waste in order to reduce the amount of land needed for its spreading or to reach discharge standards. Though anaerobic digestion may ensure substantial removal of carbon (expressed as COD and BOD₅), it leaves very high levels of nitrogen and phosphorus, and this makes attempts to complete the treatment technically and economically unfeasible. The understanding of this "bitter" truth certainly led to a decrease in the use of anaerobic digestion in animal wastes applications.
In Italy at the end of the eighties, a new generation of biogas simplified systems for animal (mainly pig) slurry has come into use, involving a simple cover made of plastic material placed over the lagoons or storage tanks (Figures 1-2). Since the majority of the Italian regional legislation prescribes the use of storage tanks - though with different retention times - the tanks were usually pre-existent to the covering. The function of the floating cover is to collect the biogas which is naturally produced under the almost totally anaerobic conditions of the tanks or lagoons and released into the atmosphere. The resulting reduction of emissions also eliminates odours.
Though no official census has been made, information gathered from the firms that produce this type of system indicates that approximately fifty of these plants have been installed in Italy up to now.
As demonstrated in experimental activity and system monitoring, the production of biogas follows the temperature curve of the slurry in the tank or lagoon.
 

Figure 1 - Scheme of a low temperature lagoon digester.

Figure 2 - A view of a biogas simplified plant for animal slurry.

However, there is still no standardized procedure for system design and prediction of the quantity of gas produced. The firms that produce this type of system use an empirical approach, recommending the covering of surfaces corresponding to 60 or 90 days of theoretical retention time.
Under good conditions, the production of methane gas that can be obtained is about 15 m³/year for each 100 kg of pig live weight (about 25 m³/year of biogas).
The concentration of methane in the biogas varies between 65% in summer to 80% in winter. Despite this seasonal variability in production, all the gas produced is available for use, as there is no energy consumption for maintaining the process temperature.
The best application for this type of system is in swine breeding activity, generally fattening, associated with dairies. As large quantities of combustible fuel are consumed in dairies for producing the thermal energy (steam) necessary for cheese-making, these facilities are usually able to burn all the biogas produced both in winter and summer, thus providing significant energy savings.
When the user wants to maximize biogas production and obtain stable production levels during the year, the raw waste is digested at controlled temperature, where possible at 35-37°C (mesophilic digestion).

The types of systems that can be used involve simplified reactors similar to those described above, added to which is a system for heating the wastes by means of a hot water pipe coil submerged in the tank (Figure 3).
 

Figure 3 - Scheme of a covered tank digester with heating system.

Because this is a simplified system, it is not always possible to ensure that the reactor temperature is maintained constant. For example, there are often cases in which the pipe coil inside the tank acts as a discharger of the heat produced by cogeneration, and therefore the temperature ranges from a minimum of 20-25°C in winter to a maximum in summer that may exceed 35°C.
Under good conditions, the potential production of pure methane gas is approximately 21 m³/year for each 100 kg of pig live weight (about 35 m³/year of biogas).
In mesophilic digestion, the average concentration of methane in the biogas is about 65%.
This type of digestion is usually associated with the combined production of heat and electrical energy for cogeneration, with a maximum yield of 30% in electrical energy and 60% in thermal energy.
 

2. EXAMPLES OF FARM SYSTEMS USED IN ITALY

2.1 The biogas plant of a pig farm

Figure 4 - Pig farm: plan and flow-sheet of the biogas plant.

The pig farm, located in the province of Parma in the Emilia-Romagna Region, is a large facility that offers a classic example of the organization of heavy swine production for Parma ham. The farm also incorporates a feed production plant.

The farm sheds have a capacity for 330 sows and 3205 growing and fattening pigs. The average live weight present amounts to about 330 tons.

Figure 5 - Pig farm: a view of the biogas plant.

The biogas system (Figures 4, 5, 6, 7) was created by adapting the tanks used for slurry storage. The slurry reaches a shaft from which a pump sends it to a rotating screen. The separated solids are collected in the underlying concrete platform.
The liquid part is divided into two identical flows by a hydraulic separator, and then conveyed to two parallel digestor tanks of the same size. Each digestion line is thus independent, and is composed of a tank built on-site with dimensions of 25 x 6 m in plan, an average depth of 4.5 m, and total useful volume of about 600 m³. The side walls of the digestion tank are insulated, partly with polyurethane foam (about 5 cm thick) and partly by earth embankments.
 

Figure 6 - Pig farm: a view of the biogas plant with, in the foreground, a slurry storage tank and, on the right side, the covered platform for the rotating screen.	Figure 7 - Pig farm: the cogenerator for biogas use.

The cogenerator is equipped with a parallel mains board which enables it to work in parallel with other potential cogenerators as well as with the ENEL (Italian national electrical agency).
The system began operation in the spring of 1993. Since the first months of operation, the system has been monitored to verify the biogas yield. Table 1 shows the main operative parameters of the plant found in the period from October 1994 to September 1996.
High biogas production rates were also due to the addition of a certain quantity of whey to the slurry that was in excess of the nutritional requirements of the pigs.
 
 

2.2 The biogas system of a dairy
The dairy is located in the province of Piacenza in the Emilia-Romagna Region. The dairy produces approximately 8,000 t of milk annually for making Grana Padano cheese.
The whey is used in pig feeding. The wastewater (about 60 m³/day, with an organic content of approximately 78 kg/day of COD) undergoes a purification treatment (an activated sludge aerobic system), and is emptied into surface water. The excess sludge, mixed with the pig slurry, is used for agronomic spreading.
 

Figure 8 - Dairy farm: scheme of the pig slurry and dairy wastewater treatment plants.

Figure 9 - Dairy farm: a view of the biogas plant, in the foreground the rotating screen.

The pig farm connected to the dairy fattens pigs for the production of animals weighing about 160 kg. The average total live weight present is about 270 t.
The housing is on a solid concrete floor and the external dung passage has a slotted floor. The slurry collected in the pits underlying the slotted floor is removed by mechanical scrapers and conveyed by means of an underground piping system to the shaft that lifts it to the screen.
The system used for handling the pig slurry (Figure 8 and 9) involves a line of treatment for the purposes of agronomic spreading, consisting of the following components:

- screening using a rotating screen for separating the coarse solids;
- simplified biogas plant composed of a 1,700 m³ tank with floating cover in plastic material which ensures the anaerobic conditions and acts as collector for the biogas produced.
 
The total storage capacity of the slurry is equal to production of about six months, and enables the implementation of a programme of fertilization-irrigation in compliance with agronomic criteria.
The biogas plant was constructed using a plastic sheet (in Hypalon) to cover a portion of the waste storage tank which was separated by a baffle made of prefabricated elements. This created 2 tanks which are communicating by means of an overflow passage, the first of approximately 10 x 38 x 4.7 metres (about 1,700 m³), in which the biogas system was installed, and the second of approximately 24 x 38 x 4.7 m (about 4,300 m³) used for storing the stabilized waste after anaerobic digestion.
The surface area of the cover is approximately 340 m², and it floats on the slurry by means of a series of "pockets" containing polystyrene blocks secured underneath the cover. The biogas produced is withdrawn by means of a perforated collection manifold in PVC that extends along the long side of the sheet, and is sent by means of a compressor and underground piping in PVC to the boiler of the dairy.
On this farm there is no heating utility, and the biogas produced by the anaerobic digestion system is used in the two steam generators operating on methane gas that serve the dairy, with power of 1,200 and 700 Mcal/h.
This biogas system was one of the first to be installed, in 1988. Since it was initially activated and for about two years (July 1988 - May 1990), this anaerobic digestion system was monitored in order to verify the biogas yield.
 
  

Table 3 provides the main operative parameters of the system and the average values of biogas production during the different seasonal periods. Biogas production was not measured in the winter because of the breakdown of the measurement instrument.
In conclusion, Table 4 summarizes, the energy balance and the economic analysis of the plant. The payback time of the capital invested is thus 4 years. The dairy made use of government funding equivalent to 60% of the investment.
 
 

Table 4 - Energy balance and economic analysis of the dairy biogas plant.
- Pig live weight	270	(t)
- Digestion covered volume	1,700	(m3)
- Covered surface	340	(m2)
- Energy saving (i.e. methane)	21,000	(m3/year)
- Economic value of energy saving	5,340	(US$/year)
- Investment cost (1988)	22,200	(US$)
- Pay back time	4.2	(year)
Note:  - 1 US$ = 1780 Italian Lire (ITL)  - We considered a methane cost of 474 ITL/m3 (0.266 US$/m3)  - The investment cost includes: the plastic cover, the compressor for conveying the biogas to the boilers, the piping and valves for hookup to the boilers and the installation; excluding the concrete tanks, which had to be constructed in any case for slurry storage.

 

3. COGENERATION-SETS WITH BIOGAS FROM PIG SLURRY
Typical electrical output of cogeneration sets in agricultural field is less than 100 kW. This is due two main factors:

• farm electrical power and energy consumption are limited;
• biogas yield is limited.

4 REFERENCES
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