Monitoring and analysis of biogas output from decentralized anaerobic waste water treatment with simultaneous utilization of resources in Java, Indonesia

The project region Gunungkidul is characterized by an acute water shortage which greatly affects the population, especially during the dry season, which lasts from April to October. The peculiarity of Gunungkidul is that despite high rainfall of about 1,800 mm per year, the water supply of the population is not guaranteed. Because of the karstic underground, rainfall rapidly seeps into the ground without sufficient surface storage (BMZ 2008). Furthermore, this region is considered to be one of the poorest areas in Indonesia. On the basis of a comprehensive set of criteria a pilot village called Pucanganom was selected which is situated in the catchment and supply area of Goa Bribin (an underground river system) and is directly connected by a sinkhole to the karstic aquifer. Through the sinkhole waste and sewage directly reaches the water resource resulting in an urgent risk situation because of the contamination of the groundwater.

The subsequent pages will discuss the detailed information on the system and the biogas production. Furthermore, the results of monitoring and analysis of the decentralized anaerobic waste water treatment system are presented.

Sanitation systems in Gunung Kidul consist of, either pit latrines (32%) or pour flush toilets (66%). Untreated liquid phase infiltrates into the ground and 80% of septic tanks are hardly ever emptied thus posing a significant threat to groundwater quality (Müller 2009). The current situation results in an inadequate waste water treatment. A lot of data were collected to get an overall picture of this region, e.g. socio-economic situation of population from personal interviews, detailed mapping of the existing waste water situation, their state and more facts. After analysis of all data gathered, the region appears to be highly vulnerable, so that some villages need urgent support to be able to change their current living standard. One main concern is healthcare. Health benefits are attained by improving water supply, sanitation and hygiene. Considering local conditions and the five sustainability criteria of sustainable sanitation alliance (SuSanA) a set of indicators were developed by scoring model (Frischknecht & Schmied 2009) to select the most appropriate waste water treatment concept.

The results of this decision-making demonstrated that socio-economic aspects such as the acceptance of the population played a major role in the selection of the concept. Based on the five sustainability criteria of SuSanA several objective criteria for example high acceptance, simple construction and less maintenance were established. Using the Scoring Model, 11 indicators such as costs, flexibility and efficiency of the system were developed. Furthermore, it was determined that the best system for this region is a semi-centralized anaerobic system by closing the nutrient and water cycle as well as producing renewable energy by treating black water together with cow dung and bio waste. Furthermore it is being discussed to use a gen-set that burns the gas to produce electricity. Even though the experience with common gen-sets in small scale makes this technique not applicable to simple low-cost digesters, there are promising biogas gen-sets available that could fill this gap. Because of H₂S inside the gas-stream, corrosion would destroy the gen-set. To avoid this, a filter needs to be installed.

For the production of sludge, cow manure as well as human faeces, night-soil and bio waste will be used. Precedes the digester there is a mixing unit which is used for mixing the cow manure with water. The recommended ratio manure to water is 1:1, so one bucket water is added to one bucket of cow manure. After mixing both components, the liquid manure is introduced into the digester. The digester unit is a water- and gas-tight chamber where an anaerobic reaction occurs. One end product of this reaction is biogas which can be delivered to households with a simple gas pipe system. The second end product is the so-called digestate and would be treated in the sludge bed. In the sludge bed, the digestate is separated into a solid and a liquid phase. This makes transport easier and also decreases the amount of pathogens. The solid phase is a perfect fertilizer and should be used, due to hygienic reasons, for fodder plants.

Biogas can replace fuel or firewood for cooking. Once the plant is working, supply of gas is free of charge and directly supplied to the household by a simple gas pipe network.

The output of the biogas plant is a nutrient rich fertilizer with very good properties.

Organic material can be used as an input for the biogas digester and therefore no service has to be provided and paid.

The biogas plant can treat black water and reduces therefore waterborne diseases and economic losses caused by soil or water contamination.

With the benefit of the biogas and a direct fertilizer for the agriculture, the people will participate more in the concept and there is a larger inducement for them to understand the current problems and future improvements.

Three pilot plants were successfully implemented in a so-called cluster mode in the project village of Pucanganom as you can see in Figure 2. The realisation of each plant took about two months including earthworks, formwork as well as gas pipe network shown in Figure 3. The construction of the digester were performed by locally available materials and existing workers. Two digesters were made of armoured concrete and one is a brick construction. The two digesters build in armoured concrete posed a challenge to the local workers as they were not used to work with armoured concrete. The brick construction in contrast proved to be easier and faster to build than the concrete unit. Locally available clay stones were used as base material.

The addition of black water as part of a co-digestion to the biogas plant was difficult as local people want to avoid contact with sewage water. The first plant was not connected to any toilet due to technical issues. The second plant is only connected to one toilet which has been running since then without any issues. To the third plant, five toilets were connected. At the beginning, plant users refused to use the biogas because they feared contaminations of the biogas. However, after a while the biogas was used without any complaints. This was probably due to the fact that the sewage pipes were installed underground and secondly because of the good performance of the sludge bed which does not remind the users in any way of sewage.

The substrate and the combination of different substrates affect strongly the performance and success of a biogas plant. By controlling the substrate added and observing the development of the gas production on the gas metre, the owner of the biogas plant (villager) can found out the best combination of substrate to get the efficient quantity and quality of gas production. In addition, they can also influence the properties of the digested sludge. Many materials can be used theoretically as a substrate for anaerobic digestion but they have to fulfil the following criteria:

• Meet the nutritional criteria stated before.

• Must include vitamins and trace elements.

• Ratio of C:N (carbon to nitrogen) should be in range between 15 and 25 (Liu 2008).

• Total solids value (TS) should be around 7–10% (Nallathambi Gunaseelan 1997).

Joncic (2012) analysed the cow manure in the region of Gunungkidul with the result of a TS value of 19%. To achieve an optimum TS value, one bucket of water should be added to one bucket of cow manure (Nallathambi Gunaseelan 1997). A very important factor of the substrate is the C:N ratio as it strongly influences the performance of the plant. In Table 1 it is shown that the C:N ratio of cow manure (6–20) is theoretically in the range of the optimum substrate (15–25) but it is recommended to do an examination of the ‘regional’ cow dung. The C:N ratio depends mostly on the food that the cow receives. Sewage has a C:N ratio of 5–11 which is low and thus not included the optimum C:N ratio. This could lead to problems for the biogas process (e.g. ammonia inhibition) if the local cow dung also has a low C:N ratio.

As different substrates represent different feasibility for the biogas process, also the gas production from the different substrates differ. The possible methane yields from sewage sludge is around 160– 350 m³ CH₄/t volatile solids (VS) and from Manure it is ca. 100– 300 m³ CH₄/t VS (Schnürer & Jarvis 2010).

With regards to increase the theoretical methane yields, co-digestion is used, i.e. the use of different substrates in the same digester. By recalling Table 1 one can recognize the first advantage of co-digestion: improving C:N ratios by mixing different substrates and therefore biogas production can be easily increased. It even happens that methane yield from co-digestion is higher than the theoretical yield sum of the singular substrates (Schnürer & Jarvis 2010). This is due to the fact that more trace elements and a better C:N ratio improves the bacteria's living conditions. Moreover, the biogas process becomes more stable because a lot of different bacteria are present in the digester and therefore the microbiological community is capable of dealing with different substrates more easily. Also, toxics are less dangerous because they will probably only eliminate one type of bacteria and other micro-organisms can replace them (Schnürer & Jarvis 2010). Co-digestion is therefore an efficient strategy to improve reactor efficiency and stability.

Based on the investigations from Joncic (2012) it is recommended to mix water, black water as well as cow manure in a certain ratio. Here, we had to estimate the daily manure production of the cow and the daily black water outcome from the households. Organic waste is not considered as it is fed to the cows in most of the cases. Sasse (1989) provided a value of 8–15 kg of manure produced by one cow per day. The value of 8 kg per day was confirmed by a local measure survey taken out in the project village of Pucanganom. The amount of cows per household is assumed to be one cow per family in the Gunungkidul region. The second feeding stock is black water from the private toilets. The WHO proposed for the production of human faeces in tropical regions and a diet vegetarian the value of 1.4 l per person and day (Franceys et al. 1992). A summary for the input values can be found in Table 2.

Two households (3.5 persons per household) should be connected to the system to respect economic criteria. The daily slurry volume is then calculated with 70 l/d. The calculation is based on the values given in Table 2.

In a next step, the daily gas production was estimated. As the plant is working in co-digestion, it is very difficult to make any predictions for the gas production. However, a rough estimation can be done by simply summing up the single contributions from the substrates. The gas production from cow manure is estimated by Sasse (1989) to be 34 l of biogas per kg cow manure. This will lead for our digester (16 kg) to a production of 544 l of biogas per day. Farmers in Gunungkidul use fresh fodder which leads to lower gas yields as when mixed feeding is applied (Møller et al. 2004).

As storage volume, Sasse (1989) suggests 40–60% of the daily production. The storage volume needed is therefore 400 l.

As the three pilot plants are now up and running, measurements were taken out in order to understand the effectiveness and reliability of the digester. To understand how the biogas plants really work it is necessary to answer some questions:

1. How much substrate and what kind have to fill in the biogas plant and are there any other substrates to increase the biogas output?

2. How much is the biogas production in the individual biogas plants?

3. How much gas is available for cooking in each household?

4. What kind of parameters have to measure?

5. What about quality issues of input, output and liquid phase in every biogas plant?

To overcome the theoretically approach and to answer the questions above a field study had taken place in Pucanganom in January/February 2014. The target was to test the surrounding conditions such as temperature and pH-value, to measure the amount of in- and output as well as to analyse the produced biogas. All results showed exemplary on Biogas plant 2. Two households, 3.5 cows and one toilet are connected to biogas plant 2.

Initially, the amount of input and output were measured. A normal bucket (0.45 kg) usually used in the village was used on the measurement of input and output. As already explained in the section substrates for biogas production the input consist of back water, cow dung and mixing water. One bucket of water (8.12 kg) added to one bucket of cow dung (9.155 kg) mixed with black water. The total volume is depending on the amount of cows per household and biogas plant. For example in biogas plant 2 the average amount of added cow dung is 32, 04 kg per day (5 buckets - 3.5 cows).

The successful implementation is shown in Figure 4, and the produced gas is used for cooking. The calculated production for one system is 647 l gas per day. With this amount of gas, it is ensured that one household can cook theoretically up to 4 h per day with a gas consumption of 80 l/h per gas stove as presented in Figure 5. This is according to their daily needs.

Within the framework of the field study, one aim was to increase the production of biogas. In the second phase which begins on 14 January/2014 organic material with an increasing ratio over 4 weeks was added. The organic material is the locally available banana tree shredded into small pieces shown in Figure 7.

To measure the gas yield and the composition of the biogas the BM2000 and the gas metre shown in Figures 8 and 9 were used. All relevant data were collected and is actually in the evaluation process.

In total 19 samples from the input, 14 from the output and 7 from the liquid phase in the sludge bed were collected. These samples were analysed in the laboratory and the evaluation is still in process.

Even though the project is running until the end of 2014 the three biogas plants were handed over to the villagers and will be operated by them from now on. The following advantages are achieved:

• Biogas replaced fuel or firewood for cooking.

• Improved sanitation situation and recycling of bio waste.

• Digestate as direct fertilizer for the agriculture. This fertilizer is free of contaminants, thus conserving the water resources.

• Biogas production is sufficient to supply a cooking site in every household for more than three hours a day, especially after the addition of shredded banana tree.

However a monitoring programme will be established to assess the efficiency of the digester and other possible additional substrates will be tested to find out a supplementary source for increasing the amount of biogas and to extend the duration time of cooking.

The authors would like to express appreciation for the support of the Project to the German Federal Ministry of Education and Research (BMBF).