Percolation is the important process of extracting the soluble constituents of a fine mesh, porous substance by passage of a liquid through it. In this study, bio-wastes were percolated under various conditions through continuous percolation processes, and the energy potential of percolate was evaluated. The representative bio-wastes from the K composting plant in Darmstadt, Germany were used as the sample for percolation. The central objective of this study was to determine the optimal amount of process water and the optimum duration of percolation through the bio-wastes. For economic reasons, the retention time of the percolation medium should be as long as necessary and as short as possible. For the percolation of the bio-wastes, the optimal percolation time was 2 hr and maximum percolation time was 4 hr. After 2 hr, more than two-thirds of the organic substances from the input material were percolated. In the first percolation process, the highest yields of organic substance were achieved. The best percolation of the bio-wastes was achieved when the process water of 2 L for the first percolation procedure and then the process water of 1.5 L for each further percolation procedure for a total 8 L for all five procedures were used on 1,000 g fresh bio-waste. The gas formation potentials of 0.83 and 0.96 Nm3/ton fresh matter (FM) were obtained based on the percolate from 1 hr percolation of 1,000 g bio-waste with the process water of 2 L according to the measurement of the gas formation in 21 days (GB21). This method can potentially contribute to reducing fossil fuel consumption and thus combating climate change.
Over 8 million tons per year of bio-waste were collected in Germany over the last decade. The collected bio-waste from households is mostly treated in compost plants. Germany has about 800 compost plants compared to approximately 80 digestion plants, which are used for biogas production from bio-waste. Classic composting has a negative energy balance because the overall energy consumption involved in composting is very high . As a result, attempts to generate renewable energy have been made at composting plants since the 1980s .
At composting plants, renewable energy can be easily obtained through the digestion of percolate from the percolation process of the bio-waste. Percolation is a process that is already used for bio-waste treatment. The objectives of percolation can also include an increase and acceleration of biodegradation, reducing treatment times, reducing the required reactor volumes, and increasing the gas yield .
The history of the development of the percolation process originated with a two-stage, two-phase biological process developed by Gosh in 1978 to enable a more rapid breakdown of organic matter in landfill reactor. In the process of percolation, the solid waste is subjected to anaerobic hydrolysis in the first stage. The resulting soluble organic substances will be absorbed by the added aqueous phase and converted to biogas in an anaerobic reactor. The purified percolation water is then returned .
The percolation process was further developed and optimized with organic waste by Rijkens  and Hofenk et al. . As starting materials, vegetables, and fruits as well as slaughterhouse waste were used, and this was implemented twice on an industrial scale in Ganderkesee with about 3,000 ton/yr of organic waste, where it was referred to as the ANM method, and in Breda as the Prethane-Rudad method implemented with vegetable waste.
In the 1980s, further experiments on two-stage, two-phase biological treatment of organic waste (fine mist, market and slaughterhouse wastes) were performed by Wellinger and Suter  and by Widmer et al. . The extraction performance of the percolation of biogenic waste, and as a result, the biogas yield, could be increased by a quasi-dynamic, ventilated operating in percolation reactor.
The first mobile demonstration plant with a capacity of 500 tons/yr was installed in Ravensburg in 1997. After that, the bases for the construction of two demonstration plants with a treatment capacity of 25,000 tons/yr in Buchen county landfill Sansenhecken and Ringsheim landfill Kahlenberg were established, and these have been operating since 2000 .
Although composting has long been applied and established for the recycling of separately collected bio- and green-wastes, the procedures are continuously optimized and provide a high level of technological security. Due to the continual ongoing quality control of the Federal Community of Goods Compost, the compost or fermentation products of 430 compost plants and 79 fermentation plants are also well secured in the RAL (State Commission for Delivery)-Quality Assurance .
Nevertheless, the importance of energy recovery with the Renewable Energy Sources Act (Erneuerbare Energien Gesetz [EEG]) and the requirements for composting are increasing due to the regulation of Technical Installation Air. This trend is expected to continue in the future.
As a solution to this problem, the designs should be reinforced with the EEG for the further development and the expansion of composting facilities with a fermentation step associated with the percolation as pretreatment system.
The central goal of this study was to determine the range of the optimal amount of the process water and to determine the appropriate duration of percolation through the bio-waste. For economic reasons, the retention time of the percolation medium should be as long as necessary and as short as possible. The most important objective of percolation of the bio-waste is to efficiently increase the biogas potential of the percolate before composting. For this purpose, the enrichment of the easy degradable carbons of the bio-waste into the percolate water should be done within the shortest time considering the economic point of view. By digesting the carbon enriched percolate biogas could be produced and it could be used for composting the percolation residue. In this study, therefore, bio-wastes were leached under various conditions through a continual percolation process, the energy potential of percolate from the continual percolation process of bio-wastes was evaluated, and the various parameters for successful percolation were optimized.
2. Materials and Methods
2.1. Construction and Characteristics of the Percolation Reactor in a Laboratory Scale
The percolation reactor (Fig. 1) was cylindrical and formed of transparent plastic. At the bottom of the reactor was a funnel in which the percolate was collected. The reactor had a height of 30 cm and a diameter of 14 cm. The usable reactor volume was 3.0â€“3.5 L. The capacity of the reactor was 1.0â€“1.5 kg biomass. An astral distribution is used to process the substrate with sprinkler water. The lower part of the reactor vessel holds a perforated sheet of stainless steel on which the substrate was filled. To prevent blockage due to processes of the substrate, the hole was covered with nylon material. For ease of test implementation, two identical reactors were built and operated in parallel.
2.2. Sample Preparation
A representative sample of the mixed, shredded organic waste with structural material was obtained through rejuvenation procedures (1 kg) at the K composting plant in Darmstadt, Germany. The waste contains deciduous and green waste (tree cuttings), because the experiments were conducted in autumn and winter. To produce a representative sample, the method of quartering was used. In quartering, part of the crushed bio-wastes from different places was shoveled alongside and mixed manually again. After that, it was heaped into a cone and divided potentially into four parts. The two opposite corners of the cone were removed, so that half of the sample material was discarded. This process was repeated 4â€“5 times more until the needed sample of 4â€“5 kg remained at the end. After quartering the collected bio-wastes, a representative sample could be taken. It must be ensured that the sifting can percolate down to the ground and that the lower layers are not compressed by the amount of the substrate cluster .
Table 1 presents the analytical characteristics of the bio-wastes used in this experiment. The bio-wastes consist of moisture (46.61%â€“62.01%), dry matter (39.47%â€“53.39%), organic matter (39.95%â€“61.79%), and ash (38.21%â€“60.05%).
2.3. Quantity of Process Water and Conducting of Experiments
For the measurement of the yield of the percolation, a total eight experiments were conducted (Table 2). In experiments 1 and 2, 1 L of water was used in each procedure, for a total of 7 L. In the first procedure during the other six experiments, 2 L of water were used. In experiments 3 and 4, four other procedures were carried out with 1.5 L process water for a total 8 L for all five procedures. In experiment 5, six other procedures were carried out with 1.5 L process water for a total of 11 L for all seven procedures. In experiment 6, 7, and 8, all seven other procedures were performed with 1.5 L of water for a total 12.5 L of process water per experiment.
For economic and technical reasons, tap water was used with a temperature of about 20Â°C â€“23Â°C room temperature as process fluid. The process water volume has to be determined by calculating the water holding capacity of the percolation material. The percolation process begins when the maximum water holding capacity is exceeded.
The substance was sprayed with water continuously for 40 min. The process water had a flow rate of 0.8 mL/s. Subsequently, it was left still for 20 min until all the water running through the substance, i.e., the process water, had exceeded the water holding capacity of the substrate. Consequently, it took 1 hr to perform one procedure, and this was then repeated. Thereafter, the percolate collected in the funnel and could be deducted for continuing investigations. The outputs (percolate water and residue) were analyzed as soon as possible to avoid disturbances or changes in the results, because the microorganisms begin to decompose the organic matter very quickly. The biogas formation during the percolation process in this test was neglected because this study focuses on the yield of the percolate.
2.4. Measurement and Analysis
The pH value, the temperature, and the conductivity were measured with a Multi 350I/SET (Wissenschaftlich-Technische Werkstaetten GmbH, Weilheim, Germany). The water and the dry matter were measured according to DIN EN 12880  and the dry organic matter and the ash according to DIN EN 12879 . The water holding capacity was measured according to methods book for the analysis of compost . The measurement of the gas formation in 21 days (GB 21) was carried out partially according to DIN 38414-8 . The rapid test methods, cell-test LCK 514 for the measurement of chemical oxygen demand (COD), LCK 365 for the measurement of organic acids, and LCK 386 for the measurement of total organic carbon (TOC) (Hach Lange GmbH, Dusseldorf, Germany), were applied.
2.5. The t-test for the Consideration of the Correlation Coefficient r
Based on the correlation and regression analysis, the following correlation coefficients and coefficients of determination are available and are listed in Table 3. A statistical test was performed. The question of whether there is a connection between the percolation time and an increasing pH should be examined. Correlation coefficient (r), coefficient of determination (r2), and the number of data pairs (n) were used in the formula:
If the test statistic is greater than the table value, then there is a correlation between elevated pH and percolation time. A statistical test as described in formula (1) was also performed for the total recovered COD to detect any possible significant correlation to the increase of the total recovered COD and the percolation time.
3. Results and Discussion
3.1. Quantities of the Percolates
The quantity of the percolates after the first procedure of the percolation for experiments 1 and 2 was 695.7 and 668.0 mL (average 681.9 mL), respectively; while in experiments 3 to 8 quantities in the range of 1,668.5 to 1,734.9 mL could be achieved (average 1,698.4 mL) (Table 2). The quantities of the percolates that were produced in experiments 3 to 8 were higher than in experiments 1 and 2. This was due to the addition of 2 L of process water in experiments 3 to 8; whereas in experiments 1 and 2, only 1 L was added. The water holding capacity of 1,000 g of bio-waste in both cases was about the same, on average 301.6 mL in experiments 3 to 8 and 318.1 mL in experiments 1 and 2. The percolate volumes were almost as much as the amounts of tap water added to the bio-wastes. This means that a condition of the percolation process that must be present is a relatively coarse solid structure with a sufficient pore volume. It has to ensure that the water can percolate down to the ground and that the lower layers are not compressed by the amount of the substrate structure.
3.2. Temperature of the Percolate
The temperatures of the percolate (Fig. 2) were in the range of 21.9Â°C to 26.4Â°C. The percolation water was room temperature and for economic and technical reasons, it was not heated separately. There were no large fluctuations in temperature, except in experiment 6, which could also be a measurement error. The temperature differences in the percolation period in all experiments ranged from 0.5Â°C (experiment 4) and 2.7Â°C (experiment 8). The differences on the final temperature (final procedure with the exception of the outlier from experiment 6) and the initial temperature were calculated. In all experiments, the temperatures were lower in the first procedure and were higher towards the end. It could be caused by aerobic microbial decomposition of organic matter. But most of the temperatures of the percolate were lower than the range of 25Â°C to 35Â°C  for the optimal condition for hydrolysis.
3.3. The pH value of the Percolate
The pH values of the percolate were in the range of 5.0 to 6.6 (Fig. 3). Usually at the beginning of the composting process, pH values are low because of the formation of fatty acids, CO2, and nitrification. Nevertheless, a significant increase in pH values was determined which means that the acidification phase, caused by the anaerobic microbial degradation, has not yet started and the organic acids from the input material were washed out by percolation. In the hydrolysis, ammonia is released from organic nitrogen compounds and thus causes an increase in pH value. This counteracts the decrease in pH value caused by the drainage. The increasing of the pH values shows that the degradation of organic matter during percolation is prevented. Optimal conditions for hydrolysis are indicated as pH values in the range of 5.2 to 6.0 .
The results of the statistical test for the correlation coefficient are shown in Table 3 and show a significant correlation between the time and the increase of pH values at a 1% confidence level in experiments 2, 5, 6, 7, and 8 and at a 5% confidence level in experiments 3 and 4.
3.4. Conductivity, TOC, Organic Acids, and COD of the Percolate
Due to the percolation, the conductivity of the percolate fell steeply (Fig. 4(a)). This means that the conductive ions can be solved quickly through a washing out in percolate. Continuing, it shows the avoidance of further biological degradation, and also serves to avoid oxidation, hydrolysis and acidification, and other chemical processes. It was in the range between 0.8 and 7.0 mS, and the differences in conductivity between the first and the last procedure in all experiments were in the range between 2.6 mS (experiment 7) and 4.2 mS (experiment 4). In all experiments, the conductivity values in the first procedure were higher, and were lower towards the end.
The highest TOC value was measured in experiment 8 (after 1 hr) and amounted to 3,952.4 mg/L; whereas the lowest TOC value was measured in experiment 7 (after 8 hr) and amounted to 186.0 mg/L (Fig. 4(b)). At the beginning of the experiment, the TOC values were always higher and towards the end, they were lower. The average TOC (67.5%) after 2 hr percolation was obtained based on the total TOC after 8 hr percolation. In addition, the average TOC (84.0%) after 4 hr operation was achieved based on the total TOC after 8 hr percolation.
Organic acids in the percolate were measured with concentrations in the range of 4,432.0 mg/L (experiment 4, after 1 hr) and 144.7 mg/L (experiment 7, after 8 hr) (Fig. 4(c)). After a 2 hr percolation period in all experiments, approximately two-thirds of the organic acids (65%) were washed out in the water percolation. The leaching of organic acids decreased dramatically after 3 hr. This means that acidification already occurred before the arrival of waste or during its storage period in the composting plant. Therefore, a certain concentration of acids was found at the first percolation. The resulting organic acids were decomposed rapidly, and were leached quickly by the first percolation.
The COD of percolate was determined with values in the range of 380 mg/L (experiment 1, after 7 hr) to 16,110 mg/L (experiment 4, after 1 hr) (Fig. 4(d)). The average COD of all experiments after a percolation period of 2 hr is approximately more than two-thirds (70.8%) of the total amount of the COD after 8 hr. The COD of percolate based on the fresh weight of the input material was determined as being in the range of 6.2 g/kg FM (experiment 1, after 1 hr) to 39.1 g/kg FM (experiment 4, after 5 hr) (Fig. 5). On average, during the first two operations, 65.1% of the total amounts of the recovered COD was also percolated. In addition, from the fourth round only low levels of COD were determined. On average, 80.2% of the total recovered COD could be achieved after the fourth operation relative to the total content.
Table 4 showed each COD load calculated from the COD values of the percolates measured every hour over 8 hr in experiments 6, 7, and 8. The total recovered COD was converted to the organic matter contained in the input material and the process water volume. The total recovered COD to 8 hr percolation rate in the medium was 33.9 g/kg FM. The conversion to the organic matter (OM) is 137.3 g/kg OM. Based on the process water a total recovered COD of 2.71 g/L was calculated.
The results of the statistical test for the total recovered COD are shown in Table 3. There was a significant correlation between the time of percolation and the increase of the total recovered COD at a 1% confidence level in experiments 1, 2, 5, 6, 7, and 8 and at a 5% confidence level in experiments 3 and 4.
To determine the correct percolation time, an evaluation of the measured COD was made. The determination of COD, which is an important value to study the yield of percolate, may enable the quantification of organic matter in the percolate. The COD or the TOC is an indicator for the biogas potential. The yield of organic substance was the highest at the beginning of the process. In the further steps of the process, the amount of COD was reduced and the course of this curve remained almost constant (Fig. 4). The total recovered COD showed also a sharply increasing curve for initial 2 hr of percolation, and then a steadily increasing curve thereafter (Fig. 5). This point from which the value has been reduced significantly shows the optimum percolation time. The optimum percolation time is a time period in which the transfer of organic matter of fresh organic waste into the percolate takes place effectively. Not only does the continuous flow of water wash out organic matter, but physical processes are also responsible for the leaching. In experiment 4, the highest total recovered COD was achieved, and after the fourth percolation procedure more than approximately 90% based on the total COD value of 5 hr was obtained (Fig. 5). The same was observed in the other experiments, and so we drew the conclusion that 4 hr is sufficient as a maximum percolation time. But for economic reasons, the percolation time should be as long as necessary and as short as possible. The concentrations of all analyzed parameters of percolate fell very steeply within 2 hr (Fig. 4). When 2 L of the process water was added to 1,000 g fresh bio-wastes at the first percolation procedure, and thereafter, 1.5 L of the process water for each further percolation procedure, more than two-thirds of the total COD value percolated for 8 hr was obtained during the first two procedures. Therefore, 2 hr was selected as the optimum percolation time for the percolation. As a result, It was found that experiment 4 was the best percolation method, in which the 2 L process water for the first percolation procedure and then the 1.5 L of water for each further percolation procedure for a total 8 L for all five procedures were used on the 1,000 g fresh bio-waste (see Fig 4). For the application of anaerobic technology, a minimum concentration of 1.5 to 2.0 g COD/L  was achieved with 2.71 g COD/L process water (Table 4).
3.5. Gas Formation of the Percolate
The gas formation of the percolate depends on the COD concentration of the percolate, as the proportion of biodegradable organics is a factor that limits the gas yield. To study the formation of the biogas, percolate samples that showed high COD concentration were selected. With the percolate, which contains organic material, biogas can be produced by chemical degradation. To investigate the gas formation potential gas production for 21 days was carried out 2 times with the percolates from experiments 6 and 7. The results are shown in Table 5 and Fig. 6.
A summary of the results based on eight procedures is presented in Table 6. With a volume of 12.5 L of process water and an input of 1 kg bio-waste, it could be determined within the percolation experiments that, the optimal percolation time was 2 hr and the maximum percolation time was 4 hr. The average values of total recovered COD based on the organic substance and the amount of added water from experiments 6, 7, and 8 after 8 hr percolation were 137.3 g COD load/kg DOM (dry organic matter) and 2.71 g COD load/L process water, respectively (Table 4). The average efficiency of percolation was 4.23 g COD/kg FM/hr, 17.16 g COD/kg OM/hr. and 0.34 g COD/L process water/hr (Table 6). The biogas potential of 11.85 NL (normal liter)/kg OM was achieved by continuous percolation for 3 hr in experiment 6 (Table 5).
K composting plant processes approximately 13,000 tons of bio-wastes annually. This would be a total recovered COD of 222.3 tons received annually, and thus could create approximately 12,478 Nm3 biogas in a year. Of this, at least, 8,235 Nm3 methane would be available annually because methane content in biogas was more than 66%. The calculation of the COD load and the biogas potential for a year relates to the data on experiment 6 based on the percolate from 1 hr percolation of 1 kg bio-waste with the process water of 2 L (Table 5).
The bio-wastes were leached under various conditions through the continual percolation process and the energy potential of percolate from it was evaluated. To achieve it, the percolation tests were conducted with different quantities of process water (1 and 2 L at the first procedures and thereafter with the quantities of 1 or 1.5 L for each further procedure). The percolation was repeated 5 or 8 times for each experiment. It took 1 hr for each percolation procedure. The percolate volumes were almost as much as the amounts of tap water added to the bio-wastes. The temperatures and the pH values of the percolate were in the range of 21.9Â°C to 26.4Â°C and pH 5.0 to 6.6, respectively. The concentrations of all analyzed parameters of the percolate fell very steeply within 2 hr, then were reduced gradually and remained almost constant after 4 hr. When 2 L of the process water was added to 1,000 g fresh bio-wastes at the first percolation procedure, and thereafter 1.5 L was added for each further percolation procedure, more than two-thirds of the amounts of the total recovered COD were percolated during the first two procedures. Therefore, 2 hr was selected as the optimum percolation time. In experiment 4, after the fourth percolation procedure approximately more than 90% based on the total recovered COD value of 5 hr was obtained. The same was observed in the other experiments, and thus we drew the conclusion that 4 hr is sufficient as a maximum percolation time. In the first percolation process, the highest yields of organic substance were achieved. The best percolation of the bio-wastes was achieved when the process water of 2 L for the first percolation procedure and then the process water of 1.5 L for each further percolation procedure for a total 8 L for all five procedures were used on the 1,000 g fresh bio-waste. When 12.5 L of the total process water was used for percolation, the average COD load of 33.85 g/kg FM, 137.29 g/kg DOM, and 2.71 g/L process water was observed. The average yield, which was based on the percolation time, was 0.34 g/L process water/hr as COD load. The gas formation potentials of 0.83 and 0.96 Nm3/ton FM were obtained based on the percolate from 1 hr percolation of 1 kg bio-waste with the process water of 2 L according to the measurement of the gas formation in 21 days (GB21).
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