USING BIODEGRADABILITY TO ASSESS THE PERFORMANCE OF SEWAGE IN ORDINARY PIT LATRINES

Biodegradability of sewage in ordinary pit latrine depends on deposition time and and layer depth. This research investigated using biodegradability of sewage in ordinary pit latrines to assess pit latrine performance for better design. The objective of this study was to measure both aerobic and anaerobic biodegradability of pit sludges to assess performance of such pits. Samples of fresh faecal sludges collected at six different vertical layers (surface, 0.2m, 0.4m, 0.6m, 0.8m and 1.0m depths) from pit latrines in selected households in Aku community using designed sampler. These samples were analysed in the laboratory for their physico-chemical and biological characteristics. Samples were taken across each pit layer to allow for heterogeneity of the contents. Collection was carefully done to ensure that bioactivity of the samples was least altered in the process. The samples were analyzed for their physico-chemical and biological characteristics namely moisture contents, temperature, volatile solids, BOD, COD, total solids, plate count, pH, suspended solids and phosphorous content using standard methods of measurement (ALPHA, 1998). Biodegradability of faecal sludge in the pit latrines was determined using the result of parameters from laboratory analyses. The result showed that in terms of BOD, twelve (12) pits had biodegradability of at least 80%. In terms of COD and SS, nine (9) pits had biodegradability of at least 80%. For the VS, eleven (11) pits had biodegradability of at least 80%. Ten (10) pits had ratio of VS: TS ranging from 0.90 – 2.93. Using COD: BOD ratio, eleven (11) pits had values ranging from 0.90 – 1.34 indicating that the pits sludges were readily biodegradable. Lower values showed lower organic contents and lower biodegradability. Result showed that Pit 6 had pH value of 7.98 at the surface and 6.42 at the bottom; Pit 11 had pH of 7.93 at the surface and 6.85 at the bottom. Pit 4 had pH of 7.77 and 8.77 at the surface and bottom respectively. The highest pH measured in this study was 8.96 and not too high to inhibit anaerobic process. In terms of BOD, 12 pits had biodegradability ranging from 80 - 97%. In terms of COD, Pits 9 and had biodegradability of 80% and 87.95% respectively. COD to BOD ratio ranged from 0.90 to 1.34. In terms of total solids, Pits 5, 8, 15 had biodegradability of 70% and above. In terms of volatile solids, 11 pits had biodegradability above 80%. TS: VS ratio ranged from 0.67 - 0.91. Temperature range was 51.67 - 66.67oC. Biodegradability in terms of BOD could not be used to effectively assess the performance of pit sludge at the lower part of the latrine since aerobic biodegradation prevailed at the surface and minimally down the profile. COD should be adopted to assess the biodegradability and performance of any pit latrine including user-education. data were analyzed using figures and model for biodegradability measurement.普通坑式厕所（pit latrine）内粪便污水的生物降解性能取决于沉积时间与层深。本研究旨在通过普通坑式厕所内粪便污水的生物降解性能，评估该类厕所的运行效能，以优化其设计方案。本研究的核心目标为测定坑式厕所污泥的好氧与厌氧生物降解性能，以此评估该类厕所的运行状况。研究人员通过定制采样器，从阿库社区（Aku community）选定住户的坑式厕所中，于6个不同垂直深度层（表层、0.2m、0.4m、0.6m、0.8m及1.0m）采集新鲜粪便污泥样本。随后在实验室对这些样本的理化与生物学特性开展检测分析。为覆盖粪污基质的异质性，采样覆盖了厕所坑内的每一层。采样过程严格规范，尽可能降低样本生物活性的损耗。本次检测依据标准测定方法（ALPHA, 1998），对样本的多项理化与生物学指标进行分析，包括含水率、温度、挥发性固体（Volatile Solids, VS）、生化需氧量（Biochemical Oxygen Demand, BOD）、化学需氧量（Chemical Oxygen Demand, COD）、总固体（Total Solids, TS）、平板菌落计数、pH值、悬浮固体（Suspended Solids, SS）及总磷含量。基于实验室分析得到的各项参数，研究人员测定了坑式厕所内粪便污泥的生物降解性能。分析结果显示，以生化需氧量（BOD）为评价指标时，12座厕所的污泥生物降解率不低于80%。以化学需氧量（COD）与悬浮固体（SS）为评价指标时，分别有9座厕所的污泥生物降解率不低于80%。针对挥发性固体（VS）指标，11座厕所的污泥生物降解率不低于80%。另有10座厕所的挥发性固体与总固体比值（VS:TS）介于0.90至2.93之间。通过化学需氧量与生化需氧量比值（COD:BOD）分析，11座厕所的该比值介于0.90至1.34之间，表明其污泥具有良好的易生物降解性。该比值偏低则对应污泥有机质含量与生物降解性能均较低。具体来看，6号厕所表层pH值为7.98，底层为6.42；11号厕所表层pH值为7.93，底层为6.85；4号厕所表层与底层pH值分别为7.77与8.77。本次研究测得的最高pH值为8.96，该数值未过高至抑制厌氧降解过程。以生化需氧量（BOD）为指标时，12座厕所的污泥生物降解率介于80%至97%之间。以化学需氧量（COD）为指标时，9号厕所与另一座（原文未明确编号）厕所的污泥生物降解率分别为80%与87.95%。化学需氧量与生化需氧量比值（COD:BOD）介于0.90至1.34之间。以总固体（TS）为指标时，5号、8号及15号厕所的污泥生物降解率达到70%及以上。以挥发性固体（VS）为指标时，11座厕所的污泥生物降解率高于80%。总固体与挥发性固体比值（TS:VS）介于0.67至0.91之间。样本温度区间为51.67℃至66.67℃。由于表层以好氧降解为主，且沿坑深向下好氧降解程度逐渐降低，因此以生化需氧量（BOD）为指标无法有效评估厕所底层污泥的运行效能。因此建议采用化学需氧量（COD）指标，评估各类坑式厕所的生物降解性能与运行效能，相关结论亦可用于面向使用者的宣教工作。本次研究通过图表与生物降解性能测定模型对数据进行了分析。