and the Efficacy of Pit Latrine Additives - Water Research Commission

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10.00 8.00 6.00 4.00 Volume of gas produced (Nml/gCOD) % moisture (g H 2 O/ g sample) 76 86 90 0 1.8 3.6 5.4 7.2 9 Amount of alkalinity added (mg NaHCO3 / g dried solid) 2.00 0.00 Figure 3.6: Combined results of cumulative gas production after 40 days from Set 1 experiments 80.00 60.00 40.00 Volume of gas produced (Nml/gCOD) % moisture (g H 2 O/ g sample) 76 82 86 88 90 91 0 1.8 3.6 5.4 7.2 9 Amount of alkalinity added (mg NaHCO 3 / g dried solid) 20.00 0.00 Figure 3.7: Cumulative gas production results after 36 days for experiment Set 2 showing dependence of gas production on additional moisture and alkalinity. 3.5.2.2 Conclusions from Set 1 and Set 2 experiments The purpose of these experiments was to determine whether addition of moisture and/or NaHCO 3 to samples of fresh VIP contents had a statistically significant effect on gas production rate under anaerobic 35
conditions. The gas production rate was understood to be correlated to the rate of anaerobic degradation of the VIP contents, and an enhanced gas production rate would thus be an indication of improved stabilisation rates. Experiments from Set 2 showed that VIP contents alone (with no additional water or NaHCO 3 had negligible gas production across three replicates; however, addition of water only or water and NaHCO 3 resulted in statistically significant increases in gas production. It was not possible to determine whether the addition of NaHCO 3 had any effect on gas production rate (either positive or negative) It was also observed that increasing the amount of water at the beginning of the experiment could have the opposite effect, i.e. reduced gas production rates as a result of absorption of CO 2 in the moisture (See Appendix B for details). However, after a certain amount of time, CO 2 absorption ceased as the inorganic carbon content in the aqueous phase approached saturation with respect to the headspace of the experiment. 1 Thereafter, the gas production rate (as opposed to the cumulative amount of gas produced) was greater for experiments with more added water than those with less. The conclusion of this work were that increasing moisture content of VIP contents has the potential to increase the rate of stabilisation of buried organic material in the pit. Significantly greater amounts of gas were produced per test unit for Set 2 units relative to Set 1 units. This supports the hypothesis that the generally poor performance of all treatments in Set 1 was less a function of the treatments than that the VIP contents used were already well digested and therefore inherently resistant to further degradation. This study supports the motivating hypothesis that the moisture content generally observed in VIP material is low and may be a limiting factor in the rate of anaerobic digestion that may occur in the buried pit contents. This implies that increasing the moisture content in the pit has the potential to increase stabilisation rates in the pit. These broad findings are supported by literature which state that the rate of accumulation in pits is lower in wet pits than in dry pits (Franceys et al., 1992). This study indicated that addition of NaHCO 3 resulted in no significant effect on gas production rate, thereby refuting the hypothesis that there is insufficient buffering capacity in pit latrine contents to support stable anaerobic digestion. This could be explained in the context of the general theory of processes in pit latrines (Section 3.1) in that if a significant proportion of the biological conversion of organic material in the pit latrine happens aerobically on the surface of the pit three effects would reduce the pH buffering requirement in pit latrine contents: The buffering capacity requirement for aerobic degradation is less than for anaerobic digestion since there is no accumulation of acid intermediates in aerobic degradation. There will be a smaller load of organic material requiring anaerobic digestion, and thus a smaller overall buffering requirement (than if the entire load of organic material entering the pit were considered) The process of aerobic degradation will increase the overall alkalinity of the contents of the pit latrines Finally, it may also be concluded that there is no significant benefit to adding water or alkalinity to pits wherein the material is already well-stabilised. 1 This is a somewhat simplistic explanation since the “equilibrium” in each experiment will be dynamic: it will depend on pH, headspace gas composition and pressure, relative rate of production of CO 2 and CH 4 , frequency of headspace venting and aqueous phase concentrations of a range of chemical species. 36
Page 1 and 2:
Scientific Support for the Design a
Page 3 and 4:
Obtainable from Water Research Comm
Page 6 and 7: EXECUTIVE SUMMARY 1 Introduction Ve
Page 8 and 9: A number of studies in which pit la
Page 10 and 11: in this study were undertaken on pi
Page 12 and 13: from samples were considerable lowe
Page 14: ACKNOWLEDGMENTS This report was com
Page 17 and 18: 3.5 Effect of additional moisture a
Page 19 and 20: LIST OF FIGURES Figure 1.1: Photogr
Page 22 and 23: 1 INTRODUCTION Ventilated improved
Page 24 and 25: pipe and also prevent the exposure
Page 26 and 27: 2 REVIEW OF EXISTING KNOWLEDGE ON T
Page 28 and 29: 2.1.2 Overview of aerobic degradati
Page 30: faeces entering in the prescribed f
Page 33 and 34: occur in the absence of oxygen, tak
Page 35 and 36: (a) (b) Figure 3.2: (a) photograph
Page 37 and 38: Aerobic degradation: In the presenc
Page 39 and 40: This section presents data from stu
Page 41 and 42: A practical definition was used to
Page 43 and 44: excess water ingress (e.g. due to h
Page 45 and 46: 3.4.4.1 Summary of results obtained
Page 47 and 48: A homogenous sample was collected f
Page 49 and 50: of this work is that a pit emptier
Page 51 and 52: esults from Study B indicate that b
Page 53 and 54: 300 T3 - faeces + inoculum + Gas pr
Page 55: the variation observed in the data
Page 59 and 60: conditions. Some additives in this
Page 61 and 62: their ability to metabolise mixed o
Page 63 and 64: Table 4.3: Results of field pit add
Page 65 and 66: change in actual volume of pit cont
Page 67 and 68: inwards leaving the pits and their
Page 69 and 70: 4.4 Laboratory-scale testing of pit
Page 71 and 72: Anaerobic units were closed, limiti
Page 73 and 74: The average rate of mass loss in ae
Page 75 and 76: Further, the moisture loss may in f
Page 77 and 78: These results confirm the findings
Page 79 and 80: In a programme of pit latrine empty
Page 81 and 82: The new understanding of processes
Page 83 and 84: 5.4.1.1 Performance of standard VIP
Page 85 and 86: A further observation was that pede
Page 87 and 88: 5.5.4 Maintenance It was observed t
Page 89 and 90: This study has not elucidated the m
Page 92 and 93: REFERENCES APHA, ed. (1998). Standa
Page 94 and 95: APPENDIX A: PHOTOGRAPHIC DIARY This
Page 96 and 97: Walls of pit during emptying. This
Page 98 and 99: Workers engaged in spraying solids
Page 100 and 101: APPENDIX B: STUDY INTO EFFECT OF MO
Page 102 and 103: 2.2 Gas production from each experi
Page 104 and 105: NaHCO 3 improved the rate of gas pr
Page 106 and 107:
moisture have a beneficial effect o
Page 108 and 109:
Complete chemical characterisation
Page 110:
Finally, it may also be concluded t
Page 114 and 115:
APPENDIX D: PROTOCOL FOR TESTING PI
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4.2 Chemical analyses In addition t
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