Effect of Moisture Content on the Composting Process In a Biotoilet System
By Zavala, Miguel Angel Lopez; Funamizu, Naoyuki
Biotoilet is a composting toilet that uses sawdust as a matrix for bioconversion of human excreta into compost and is managed with the aim of accelerating decomposition, optimizing efficiency, and minimizing any potential environmental problems. Understanding how the moisture content affects the biodegradation rates of feces is a key factor for setting criteria for the proper design and operation of the biotoilet. A research project in this respect was conducted in laboratory-scale composting reactors. Results showed that composting is characterized by different biological responses of microorganisms depending on the moisture content under which the process is conducted. Low moisture contents (< 64%) ensure aerobic degradation of feces, whereas high moisture levels (≥ 64%) cause both aerobic and anaerobic decomposition. Higher reductions in parameters such as total solids (TS), volatile solids (VS), and chemical oxygen demand (COD), and higher oxygen utilization rates were obtained at moisture contents near 65%. This moisture level is the critical moisture estimated during drying tests on sawdust. At high moisture contents, odors, anaerobic emissions, nitrite formation, and increase of sulphate concentrations were detected. Keeping moisture content near 60%, or little higher, but avoiding levels near or higher than 65% ensures an optimum performance of the biotoilet system.
Introduction
The biotoilet is an important subsystem of the Onsite Wastewater Differentiable Treatment System (OWDTS) for treating the toilet wastes such as feces, urine and toilet paper (Lopez Zavala et al. 2002). Biotoilet is the name of a dry or composting toilet that uses sawdust as a bulky matrix for bioconversion of human excreta into compost which can be used either as organic fertilizer rich in nitrogen (N), phosphorous (P) and potassium (K), or as a soil conditioner (Del Porto & Steinfeld 2000; Kitsui & Terazawa 1999).
As discussed in a previous paper (Lopez Zavala et al. 2005), the biotoilet system differs from conventional composting systems in several ways:
a) Human excreta are treated;
b) The composting reactor of the biotoilet system is provided with heating and mixing systems that ensure a continuous thermophilic-aerobic biodegradation process and a uniform temperature distribution;
c) The moisture content in the composting reactor is kept in the range 50-60% by heating and ventilation;
d) The system is managed with the aim of accelerating decomposition, optimizing efficiency, and minimizing any potential environmental or nuisance problems (odor);
e) Traditional composting systems have batch configurations where drying is an important process for the proper management and operation; whereas the biotoilet is a continuous feed system where the biodegradation rate of organic matter is more important because feces are daily added into the composting reactor.
Because microorganisms are essential to composting, environmental conditions that optimize microbial activity will maximize the rate of composting. Microbial activity is influenced by oxygen levels, particle size of sawdust matrix, nutrient levels and balance (indicated by the carbon-to-nitrogen ratio), moisture content, temperature, and acidity/alkalinity (pH). Any changes in these factors are interdependent; a change in one parameter can often result in changes in others. (EPA,1994; Horisawa et al. 2000, Madejon et al. 2002; Richard et al. 2002). Among these factors, moisture content has been referred to as the critical design and operating factor to optimize compost engineering systems, because aerobic decomposition of organic matter depends on the presence of water to support microbial activity (EPA,1994; Horisawa et al. 2000, Madejon et al. 2002; Richard et al. 2002).
Biodegradation rates are affected by moisture through changes in oxygen diffusion, water potential and microbial growth rates (Richard et al. 2002). Several researchers have taken the challenge of setting optimum moisture contents and their relationships in conventional composting systems (EPA 1994, 1995; Kaneko & Fujita 1985; Madejon et al. 2002; Richard et al. 2002; Suler & Finstein 1977). Only few studies have been conducted on systems similar to the biotoilet (Horisawa et al. 2000), but no research has been conducted to describe the effect of moisture content on the composting process in the biotoilet system.
The optimum moisture content represents a tradeoff between moisture requirements of microorganisms and their simultaneous need for adequate oxygen supply (Madejon et al. 2002). Water is the key ingredient that transports substances within the composting matrix and makes the nutrients physically and chemically accessible to the microbes. If the moisture levels drops below about 40-50%, the nutrients are no longer in an aqueous medium and not easy available to the microorganisms. Their microbial activity decreases and the composting process slows. Below 20% moisture, very little microbial activity occurs (EPA 1994). Optimum moisture content for biodegradation can vary widely for different compost mixtures and composting times, ranging from near 50 to over 70% on a wet basis (Richard et al. 2002). Other researchers have reported optimum moisture content in the range of 50-60% for different compost mixtures (EPA 1995; Kaneko and Fujita 1985; Suler and Finstein 1977), even others in the range 25, 30 to 80% moisture (Horisawa et al. 2000; Madejon et al. 2002). This wide range of optimum moisture content indicates 1) the complex dynamic nature of the composting process, with changes in particle size and structure occurring over the time, and ii) the necessity of more fundamental and inclusive parameter for understanding the physical and biological interactions controlling the composting process (Madejon et al 2002; Richard et al. 2002).
FIGURE 1. Schematic representation of the experimental device.
Because optimum moisture management is a key factor on the composting process and consequently in the design and operation of the biotoilet system; in this study, the effect of moisture content on the thermophilic aerobic biodegradation of feces was assessed through the quantification of reductions in TS, VS, and COD; the production of anaerobic products (volatile fatty acids) and odors; the changes in concentration of nitrification products and sulphates, and the analysis of oxygen utilization rates (OUR) and CO production rates in batch tests conducted in laboratory-scale composting reactors under several moisture content levels.
Materials And Methods
Experimental Device
As described in the previous paper (Lopez Zavala et al. 2005), Figure 1 shows the schematic representation of the experimental device (laboratory-scale composting reactors) used for conducting batch tests. Four bioreactors, constructed of glass and steel, and provided with steel porous plate at the bottom for ensuring well distribution of air supply, were placed into water baths. Sensors for oxygen, temperature and pressure were set in the inlet and outlet of the bioreactor, and an additional temperature sensor was also placed inside of it. For CO^sub 2^ monitoring, a sensor was set at the exit of the reactor. All sensors were properly connected to a computer for monitoring. Air was supplied into the bioreactor continuously and air flow rate and pressure were controlled and kept constant by using a flow meter. Before measuring oxygen concentration, air was dried with silica gel. Every reactor was provided at the exit with a flask containing a liquid solution to capture volatile fatty acids (VFA) emissions.
Batch Tests
Batch tests for six different moisture contents (from 50 to 76%) were conducted under the conditions presented in Table 1. Sawdust from a biotoilet under operation was used as a bulky material for conducting the batch tests. Feces were properly mixed into the sawdust for ensuring uniform and completely mixed conditions, adding distilled water for reaching the corresponding moisture contents. All mixtures were set up at 50C and air was supplied for each trial at a rate of 100 ml/min. Input and output oxygen and carbon dioxide (in some trials) concentrations were monitored every 30 minutes and parameters such as TS, VS, COD, volatile fatty acids (acetic acid, propionic acid and butyric acid), nitrite (NO^sub 2^), nitrate (NO^sub 3^), sulphate (SO^sub 4^), and moisture content (θ) of feces, sawdust and compost (final product) were measured in all trials. Additionally, VFA emissions were captured and measured in a liquid solution.
Total solids, VS, and θ were determined gravimetrically. COD was determined by using a standard method for water analysis with some modifications to ensure precise determination due to solid samples were used (Lopez Zavala et al. 2005). Liquid extracts of feces, sawdust and compost were prepared using distilled water; nitrites, nitrates, and sulphates contained in these extracts were determined by ion chromatographic method described in Standard Methods for the Examination of Water and Wastewater (1989); whereas, VFA in these extracts and VFA emissions captured in a liquid solution were determined by ion exclusion chromatography (DX-120, DIONEZ Corp). Batch tests were stopped approximately two weeks after starting them.
TABLE 1.
Experimental conditions set up for evaluating the moisture content effect on \aerobic biodegradation of feces by using sawdust as a matrix
Results
Effect Of Moisture Content On TS, VS And COD Reductions
Results of batch tests are summarized in Table 2, where values of parameters at high moisture contents (> 64%) are shaded. The reduction percentage for parameters such as TS, VS, and COD was calculated by dividing the values of reduction per the corresponding value of the parameter for feces, and multiplying by 100, for instance, TS reduction (%) = TS reduction (g)/TS feces (g)*100.
In general, reductions in TS, VS, and COD increase gradually as the moisture content increases up to a value near 65%. Moisture levels higher than this value cause a decreasing tendency on reductions of such parameters; however, some higher reductions are observed at high moisture contents, > 64% (Figure 2).
FIGURE 2. TS, VS, and COD reductions obtained in batch tests.
TABLE 2.
Parameters evaluated during batch tests
Effect Of Moisture Levels On Contents Of Nitrification Products (Nitrites) and Increases Of Sulphate Concentrations
Table 2 presents the measurements conducted during the experimental batch tests. At high moisture contents (≥ 64%) nitrification products (nitrites) and increases of sulphate concentrations were detected, mainly remarkable at 76%; whereas, at low moisture contents (≥ 64%) a little reduction is observed rather than an increase in concentrations of such parameters (Figure 3). In all experimental tests nitrate formation was not detected.
FIGURE 3. Nitrites formation and increase of sulphates concentration for several moisture contents.
Effect Of Moisture Content On Anaerobic Products (VFA) And Odors
Results of measurements conducted during batch tests are also shown in Table 2. Unlike the low moisture contents, anaerobic products (VFA) and odors were detected at high moisture content levels (64%). Indeed, acetic acid was detected in emissions and in sawdust matrix; acetic acid emissions captured in the liquid solution totalized 5.8 and 10.3 mg at moisture contents 64 and 70%, respectively; meanwhile, 3.2 mg [VFA] / g (dry solids) at the 76% moisture content were measured in the sawdust matrix one day after the beginning of batch tests.
Effect Of Moisture Content On OUR Profiles
To facilitate the interpretation of OUR profiles and regarding that two repetitions were conducted for each trial, Figure 4 shows the experimental OUR profiles for only one repetition. As seen, OUR profiles corresponding to 50, 60, and 76% moisture contents present only one peak; whereas, those for moisture contents 64, 65, and 70% show two peaks; the first, of higher and rapid development, occurs at 9-15 hours after starting the batch tests; the second, of wider development, happens approximately 7 days after starting the trials. Even though 64 and 65% are nearly equal moisture content levels, differences in OUR and in reductions of TS, VS, and COD are observed. Differences in peak OUR between these two moisture contents levels are probably related to the effect of moisture content of feces on response of microorganisms, because at 64% batch test (higher OUR), the moisture content of feces was 80.3%, whereas at 65% test, the moisture content of feces was 78.5%; therefore, more work in this respect is needed.
FIGURE 4. Effect of moisture content on oxygen utilization rates.
Contributions To The Oxygen Utilization Rate Profiles
Figures 5a and 5b show the OUR and CPR profiles for the 76 and 70% moisture contents, respectively. As seen, OUR is higher than CPR, especially during the early stages of the biodegradation process. Similar pattern is obtained for 64 and 65% moisture contents, but differences in magnitude of OUR and CPR are more remarkable at 76%. These differences in magnitude of OUR and CPR indicate that carbonaceous material biodegradation is not the only process that consumes oxygen during the experimental tests.
FIGURE 5. Oxygen utilization and CO^sub 2^ production rates.
At low moisture content levels (> 64%), magnitude of OUR profiles is almost similar to that of CPR profiles, especially during the early stages of the biodegradation process (Figure 5c). This figure corresponds to a moisture content of 60%, but similar pattern is observed for a temperature of 40C (result not included in this paper). Because no significant differences in magnitude of OUR and CPR are observed, carbonaceous material biodegradation is the only process that consumed oxygen during the experimental tests.
Discussion
Analysis of above results denotes that composting in the biotoilet system is characterized by different phenomena depending on the moisture content under which the process is conducted. At low moisture content (<64%), composting is characterized by an aerobic biodegradation process where:
Reductions in TS, VS, and COD increase as moisture content increases. Reasonable degradation rates are obtained at 60%. At 50% microbial activity decreases and the composting process slows; indeed, the lower OUR and the lower reductions of TS, VS, and COD during the experimental tests are observed at this moisture level. Limitations in transport of nutrients may be the reason of the drop in biological activity. Indeed, EPA (1994) reports that if moisture content level drops below 40 to 45%, the nutrients are no longer in aqueous medium and they are not easy available to the microorganisms, consequently, the composting process slows and finally the organic matter is not completely stabilized. This is clearly shown in Figures 2 and 4.
Nitrification products in the sawdust matrix are not detected and SO concentrations remain almost constant or are reduced a little as shown in Table 2 and Figure 3. Nitrifying bacteria are very sensitive to temperature, so that, this factor, among others, could be the factor responsible of nitrification process inhibition. Indeed, literature reports that nitrification process occurs over a range of approximately 4-45C, with about 350C optimum for Nitrosomonas and 35420C optimum for Nitrobacter (Eklind & Kirchmann 2000; EPA1993). Consequently, the 50C0 -temperature, set during the experimental tests, probably inhibits the growth and activity of nitrifiers.
Anaerobic emissions and odors are absent. It means oxygen reaches most inner zones in the sawdust matrix keeping a complete aerobic environment for microorganisms, which act mainly over the substrate constituted by feces. This activity or response is characterized by an OUR profile with only one peak (Figure 4).
OUR increases as moisture content increases as a result of higher availability of nutrients for microorganisms. Carbonaceous material biodegradation is a CO generating process, so that, OUR is linked to the CO2 production rate (CPR), if OUR and CPR profiles are expressed in (mmol/h), it is expected that, when plotted, both show almost same magnitude. Indeed, OUR and CPR kept almost similar levels; it means that OUR is only linked to carbonaceous material biodegradation and heterotrophic microorganisms activity. This is corroborated with the lack of detection of nitrification products and increases in sulphates concentration.
The decomposition process at high moisture contents (≥ 64%) occurs under both aerobic and anaerobic conditions, and is characterized by:
High reductions in TS, VS, and COD are observed (Table 2 and Figure 2). Indeed, at moisture levels of 65 and 70%, the reduction of VS and COD is near 90%. At these moisture content levels the sawdust matrix is partially decomposed. This partial decomposition of sawdust may be associated to the growth and activity of actinomycetes and fungi, which are capable of degrading hemi- cellulose and cellulose and lignin, respectively (Kaiser 1996). Indeed, white-rot fungi are observed during experimental batch tests. Contributions of sawdust decomposition are not included in TS, VS, and COD reductions.
Reductions in TS, VS, and COD decrease as moisture content increases. High reductions at a moisture content nearly 65% are assumed to be the result of bacteria degradation and activity of actinomycetes and fungi; however, at much higher moisture levels, oxygen does not penetrate in the inner zones of the sawdust matrix limiting the activity of these aerobic microorganisms; thus, anaerobic microorganisms take more important role in the decomposition process and consequently, lower reductions on TS, VS, and COD are obtained. Indeed, excess moisture increases the thickness of the water film that surrounds the matrix particles and fills the smaller pores between particles, limiting the oxygen transport. Oxygen constraints reduce the rate of decomposition and increase the emission of anaerobic odors (Richard et al. 2002). These limitations in oxygen transport are caused by diffusional limitations of oxygen into the sawdust matrix rather than limitations in the air supply because air is sufficiently supplied. In spite of diffusional limitations in oxygen transport, reductions in TS, VS, and COD remain quite high, which demonstrates that both aerobic and anaerobic processes take place during the composting process.
Nitrification products (nitrites) and increases of sulphate concentrations are detected (Table 2 and Figure 3). Even though literature reports that nitrification process occurs over a range of approximately 4-45C, with about 35C optimum for Nitrosomonas, responsible of converting ammoniacal nitrogen into nitrites, during our experimental tests conducted at 50C nitrite formation is detected, especially remarkable at the higher moisture content (76%). Surely, more research is needed to have more conclusive results in this respect because quantifying the temperature and moisture content effects on nitrification process in composting systems is difficult (EPA 1993).
On the other hand, sulphate concentrations increase because sulphur bacteria oxidize aerobically the hydrogen sulphide (or other reduced-sulphur compounds) produced duri\ng anaerobic decomposition in those inner zones of sawdust matrix where oxygen can not penetrate.
Odors and VFA, in emissions and in the sawdust matrix, are a clear indication that anaerobic reactions occur under high moisture levels. Even though VFA measurements are mainly of qualitative character, the levels of detection in Table 2 show that higher moisture content results in higher anaerobic emissions as indicated by VFA detection. In addition, at moisture content levels near 65 and 70% VFA in the sawdust matrix are not detected, or only traces are measured; however, important amounts were captured in emissions. At 76% moisture, VFA are mainly detected in the sawdust matrix. These differences could be because VFA diffuse more rapidly at moisture levels lower than at 76%, consequently they escaped or were aerobically oxidized when they diffused through the sawdust matrix.
Higher OUR at moisture contents lower than 76% is a result of aerobic heterotrophic bacteria activity and oxidation of anaerobic products. The existence of a second peak may denote the aerobic oxygen consumption of actinomycetes and fungi when they act on the sawdust matrix after feces degradation, as explained above. It is clear that the second peak contributes importantly to obtain higher biodegradation rates at these moisture contents, as shown above. Because actinomycetes and fungi are aerobic microorganisms (Smith and Scott 2002), much higher moisture content, such as 76%, can limit their growth and activity over the sawdust when more severe anaerobic conditions prevail in the matrix, consequently a second peak in the OUR profile is not observed and lower reductions on TS, VS, and COD are obtained.
Carbonaceous material biodegradation is not the only process that consumes oxygen; other bacterial processes such as nitrification and oxidation of reduced-sulphur compounds also consume oxygen. Like nitrifiers, the autotrophic sulphur bacteria utilize inorganic compounds instead of organic matter to obtain energy, and use carbon dioxide or carbonate as a carbon source. Sulphur bacteria are able to oxidize hydrogen sulphide (or other reduced-sulphur compounds) to sulphuric acid (Spanjers et al. 1998). However, contribution of each process to the observed total respiration rate of the biomass is difficult to distinguish in many cases. On the other hand, carbonaceous material biodgradation is a CO^sub 2^ generating process, so that, OUR is linked to the CO^sub 2^ production rate (CPR), if OUR and CPR profiles are expressed in (mmol/h), it is expected that, when plotted, both show almost the same magnitude. However, when OUR is higher than CPR, the difference between them should be assumed as the contribution of nitrification and oxidation of reduced-sulphur compounds. Thus, the oxygen consumption associated with these processes can be quantified through the formation of nitrites and nitrates and the increase of SO^sub 4^ concentration in the sawdust matrix.
Contributions of noncarbonaceous material biodegradation processes to the respiration rate are more remarkable at much higher moisture levels, as seen in Figures 5a and 5b, where differences between OUR and CPR are more visible. These differences are a good indication and support the occurrence of nitrification process and aerobic oxidation of hydrogen sulphide (or other reduced-sulphur compounds), as shown in Figure 3.
On the other hand, biological response of microorganisms to different levels of moisture contents during composting of feces in the biotoilet system, expressed through above findings, couples interestingly and quite perfectly to merely physical properties (vapor pressure) exerted by moisture when it is found in a hydroscopic and porous substances like the sawdust matrix used in the composting reactor of the biotoilet. Figure 6 shows the drying rate curve obtained from sawdust drying tests conducted at 5O0C in our laboratory (Sugawara 2003). Regarding the theory and concepts of drying of solids, which are used to describe the moisture content of substances (Treybal 1968), in Figure 6 three types of moisture are identified, unbound moisture, bound moisture, and critical moisture. Unbound moisture refers to the moisture contained by the sawdust which exerts an equilibrium vapor pressure equal to that of the pure liquid at the same temperature. Bound moisture refers to the moisture contained by the sawdust which exerts an equilibrium vapor pressure less than that of the pure liquid at the same temperature. Bound moisture may be moisture held in small capillaries and crevasses throughout the structure of sawdust or otherwise adsorbed upon the surface of sawdust particles. Critical moisture refers to the moisture level at which the inflexion of the drying curve occurs, between unbound moisture and bound moisture. In other words, critical moisture is the minimum moisture level that exerts an equilibrium vapor pressure equal to that of the pure liquid at the same temperature. From several drying tests conducted on several particle sizes of sawdust, the average critical moisture content appeared to be nearly 65% (1.86 g/g). Nearby this moisture level the highest reductions on TS, VS, and COD are obtained and the highest OUR is monitored. This result has important implications i) aerobic and anaerobic decomposition, during the experimental batch tests, occurs under the unbound moisture conditions, i.e., unbound water is a synonym of anaerobic conditions; ii) aerobic decomposition of feces is conducted by bacteria under the bound moisture conditions; iii) critical moisture is the frontier moisture content which defines either the highest degradation rates or the beginning of odor and anaerobic emissions, increases of sulphate concentrations, and maybe of the nitrification process under thermophilic conditions.
FIGURE 6. Types of moisture and their link with the biological response of microorganisms in the composting process of the biotoilet system.
Based on above results and discussions, from the practical operation and management point of view, the composting process in the biotoilet should be conducted under moisture content conditions that:
Ensure fast and complete stabilization of organic matter of feces when these are added continuously (every day). Moisture contents, near or higher than 65%, guarantee that microorganisms conduct complete feces decomposition at high rates (high reductions on TS, VS, and COD);
Avoid problems with odor and anaerobic emissions. Moisture contents, near or higher than 65%, develop such nuisance conditions because they cause anaerobic conditions that limit the penetration or diffusion of oxygen into inner zones of the sawdust matrix, even though high air flow rates are maintained;
Ensure low necessities of maintenance and services. Moisture contents, near or higher than 65%, cause decomposition of sawdust matrix in the biotoilet system, which is undesirable because physical properties of sawdust are affected. Terazawa et al. (1995) found that sawdust deterioration is associated with increases of density of matrix and amount of small pores, increase of water retention capacity, and decrease of air retention capacity. Theses changes in sawdust properties create improper environmental conditions for the aerobic growth of microorganisms and disadvantageous operational conditions and maintenance necessities of the system because sawdust must be replaced more frequently. It is important to mention, however, that under continuous feeding of feces (every day in normal operation of the biotoilet) decomposition of sawdust may not occur because growth and activity of actinomycetes and fungi start just after main activity of bacteria diminish in the experimental batch tests (Figure 4).
Based on these assessments, optimum moisture control and management in the composting reactor of the biotoilet system should take into account not only high performance on biodgradation of feces, but also problems of odor and anaerobic emissions, and necessities of maintenance and services, associated with frequency of sawdust replacement. These requirements could be achieved by keeping moisture contents at 60% or little higher, but avoiding levels near or higher than 65%, i.e., moisture levels near but lower than the critical moisture. Very low moisture contents, less than 50%, should also be avoided to ensure proper environment for microorganisms and consequently faster and complete stabilization of organic matter contained in feces.
On the other hand, the critical moisture could be a physical reference parameter of great significance and utility for the operation and control of the composting process in the biotoilet, because the biological responses of microorganisms are closely link to physical properties (vapor pressure) that moisture content exerts in the sawdust matrix.
Conclusions And Perspectives
The composting process in the biotoilet system differs from that in conventional composting systems. Understanding how the moisture content affects the rates of aerobic biodgradation of feces in the biotoilet composting reactor is a key factor for setting criteria for the proper design and operation, and for achieving the aims of the system.
Even though several researchers have taken the challenge of setting optimum moisture contents and their relationships in conventional composting systems, they have recognized the necessity of more fundamental and inclusive parameters for understanding the physical and biological interactions controlling the composting process (Suler and Finstein 1977; Kaneko and Fujita 1985; EPA 1994, 1995; Madejon et al. 2002; Richard et al. 2002).
On the other hand, only few studies have been conducted on systems similar to the biotoilet (Horisawa et al. 2000), but no research has been conducted to describe the effect of moisture content on the composting process in the biotoilet system. Therefore, the significanceof this work is that it covers the lack of specific information related to influence of moisture content on the biological interactions that occur in a specialized and controlled composting process, and where, potential environmental or nuisance problems (odor) that could develop have a direct impact on people’s life and on affordability of the system. Thus, the main findings of this study are:
Composting in the biotoilet system is characterized by different biological response of microorganisms depending on the moisture content under which the process is conducted. Low moisture contents (< 64 %) ensure aerobic degradation of feces, whereas high moisture levels (≥ 64% ) cause both aerobic and anaerobic decomposition;
Because anaerobic conditions occur at high moisture contents (≥64%), microorganisms’ activity generates odor and VFA emissions. In addition, simultaneous aerobic and anaerobic processes at high moisture levels cause the increase of sulphate concentrations and formation of nitrites in the sawdust matrix, even though the composting process is conducted at thermophilic temperatures. At low moisture contents, anaerobic emissions, nitrification products and increase of sulphate concentrations are not detected;
At high moisture contents, nitrite formation and increase of sulphate concentrations contribute importantly to the respiration rate; however, the oxygen utilization rates decrease at much higher moisture levels. At low moisture levels, carbonaceous material biodgradation is the only contributor to the respiration rate;
Aerobic and anaerobic decomposition, during the experimental batch tests, occurs under the unbound moisture conditions, i.e., unbound water is a synonym of anaerobic conditions. Unlike, under the bound moisture conditions bacteria conduct aerobic decomposition of feces;
Higher reductions in parameters such as TS, VS, and COD, and higher oxygen utilization rates are determined at moisture content nearly 65%. This moisture level is the critical moisture estimated during drying tests on sawdust. Thus, critical moisture is the frontier moisture content which defines either the highest degradation rates or the beginning of odor and anaerobic emissions, increases of sulphate concentrations, and the nitrification process under thermophilic conditions. Reasonable degradation rates are obtained at 60%;
Optimum moisture control and management in the composting reactor of the biotoilet system should take into account not only high performance on biodgradation of feces, but also problems of odor and anaerobic emissions, and necessities of maintenance and services, associated with frequency of sawdust replacement. These requirements could be achieved by keeping moisture contents at 60% or little higher, but avoiding levels near or higher than 65%, i.e., moisture levels near, but lower than the critical moisture. Very low moisture contents, less than 50%, should also be avoided to ensure proper environment for microorganisms and consequently faster and complete stabilization of organic matter contained in feces.
Acknowledgements
We thank Mr. Toshihiro Kitsui (Seiwa Denko Inc.) who provided us with sawdust samples from biotoilets under operation.
This work has been supported by CREST of JST (Japan Science and Technology Corporation).
References
APHA, AWWA and WPCF. 1989. Standard methods for the examination of water and wastewater. 17th edition. USA.
Del Porto, D. and C. Steinfeld. 2000. The composting toilet system book. Centre for Ecological Pollution Prevention. USA.
Eklind, Y. and H. Kirchmann. 2000. Composting and storage of organic household waste with different litter amendments. II: nitrogen turnover and losses. Bioresource Technology, 74:125-133.
EPA. 1995. Composting: decision maker’s guide to solid waste management. Volume II (EPA 530-R-95-023), USA.
EPA. 1994. Composting yard trimmings and municipal solid waste. Office of Solid Waste and Emergency Response, (EPA 530-R-94-003), USA.
EPA. 1993. Manual: nitrogen control. Office of Research and Development, (EPA/625/R-93/010), USA.
Horisawa, S., Y. Tamai, Y. Sakuma, S. Doi and M.Terazawa. 2000. Effect of moisture content of a wood matrix on a small-scale biodgradation system for organic solid waste. J Wood Sd, 46:317- 321.
Kaiser, J. 1996. Modelling composting as a microbial ecosystem: a simulation approach. Ecological Modelling, 91:2537.
Kaneko, H. and K. Fujita. 1986. The moisture limit for optimum composting. Proc. Japan Soc. Civil Eng.; 369:303309.
Kitsui, T. and M. Terazawa. 1999. Environmentally-friendly toilets for the 21st century, Biotoilet. Proceedings of the 10th ISWPC, at Yokohama, 3: 120-121.
Lopez Zavala, M.A., N. Funamizu and T. Takakuwa. 2005. Biological activity in the composting reactor of the biotoilet system. Bioresource Technology, 96:805-812.
Lopez Zavala, M.A., N. Funamizu and T. Takakuwa. 2002. Onsite wastewater differentiable treatment system: modelling approach. Wat. Sd. Tech., 46(6-7):317-324.
Madejon, E., MJ. Diaz, R. Lopez and F. Cabrera. 2002. New approaches to establish optimum moisture content for compostable materials. Bioresource Technology; 85:73-78.
Richard, T.L., H.V.M. (Bert) Hamelers, A. Veeken and T. SiIva. 2002. Moisture relationships in composting processes. Compost Science and Utilization, 10(4):286-302.
Smith, P.G. and J.S. Scott. 2002. Dictionary of water and wastewater management. IWA publishing. London.
Spanjers, H., P.A. Vanrolleghem, G. Olsson and P.L. DoId. 1998. Respirometry in control of the activated sludge process: principles. Scientific and Technical Report No. 7. IAWQ, London.
Sugawara, T. 2003. Fundamental study on drying process in sawdust. Bachelor’s thesis. Department of Environmental Engineering, Hokkaido University, Japan (in Japanese).
Suler, DJ. and M.S. Finstein. 1977. Effect of temperature, aeration and moisture on CO formation in bench-scale, continuously thermophilic composting of solid waste. Appl Environ Microbiol, 33:345-350.
Terazawa, M., Y. Tamai, S. Horisawa and K. Yamashita. 1995. Ability of sawdust as an artificial soil. Biodgradation of nonlignocellulosic substances II. Div. of Env. Res., Grad. School of Agric., Hokkaido University, Japan, pp. 77-84.
Treybal, R.E. 1968. Drying. In: Treybal, R.E. Mass transfer operations. second edition. McGraw-Hill, Tokyo.
Miguel Angel Lopez Zavala and Naoyuki Funamizu
Department of Environmental Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, Japan
Copyright J.G. Press Inc. Summer 2005