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Wageningen Universiteit (2003)

Towards sustainable and robust on-site domestic wastewater treatment for all citizens

Mgana, S.M.

Titre : Towards sustainable and robust on-site domestic wastewater treatment for all citizens

Auteur : Mgana, S.M. 

Université de soutenance : Wageningen Universiteit

Grade : Doctor of Philosophy (PhD) 2003

Résumé
In most developing countries commonly practiced domestic wastewater treatment systems predominantly constitute anaerobic treatment process. The anaerobic treatment units mostly installed are on-site at residential dwellings. However the commonly installed units, viz., traditional pit latrines and septic tanks are in fact ’low-rate’ anaerobic pre-treatment units and are associated with inefficiency, poor maintenance and groundwater pollution. Moreover since most poor communities, who constitute the majority in the developing countries’ populations could afford these types of anaerobic pre-treatment units, their numbers have also grown to unmanageable proportions. Consequently the demand for effective but low cost wastewater treatment facilities for developing countries is indisputably great. On the basis of already available technical information concerning the Upflow Anaerobic Sludge Bed (UASB) reactor performance a wastewater treatment system based on the UASB reactor can lead to a compact, effective and low cost community on-site pre-treatment unit for tropical wastewaters. However the performance of these systems in an actual community on-site situation has so far not been investigated. This thesis therefore investigates the performance and feasibility of using the UASB reactor for the pre-treatment of wastewater under the conditions that arise at community level in tropical regions, viz. highly varying organic and hydraulic loads, but low variation in temperature. On-site pilot scale UASB reactors were configured and operated in parallel at community level for the purpose of acquiring performance data. All the reactors were operated in gravity flow mode at ambient tropical temperature of 25 - 34 ºC. The wastewater in the study area - community level - was highly biodegradable with an average ratio COD:BOD 5 of 1.52 at a standard deviation of 0.13. The wastewater characteristic was highly variable. A 1277-day monitoring duration of the wastewater grab samples has shown that the values of organic loads (with standard deviation in brackets), in terms of COD tot , and COD ss , were 529.4 (544.6) and 264.4 (448.4) mg/L respectively. A conventional pilot single-step community on-site UASB reactor (volume : 1.5 m 3, height : 1.7 m) was operated over three and a half years at an average hydraulic retention time of 6.2 (4.92). The performance data obtained via regular monitoring of the treatment unit showed a declining removal efficiency over time with respect to COD total , which likely can be attributed to the increasing rate at which biogas was produced along with the growth of sludge bed and the presence of floating sludge. As a result the removal of dispersed sludge particles becomes poorer, which likely is reinforced by the possible ’less’ optimal dimensions and design of the Gas-Solids-Separator (GSS) device. The average removal efficiency on COD tot basis was 64 percent. However a study of a parallel pilot two-step community on-site UASB reactor configuration gave more promising results. The two-step UASB pre-treatment unit in this research refers to two UASB reactors connected in series, viz. a first 2m high 1.8m 3UASB reactor put in front of a second 1m high 0.852 m 3UASB reactor. The second-step UASB reactor is a recipient of effluent including washouts from the first-step UASB reactor The average organic loads of the wastewater imposed to the system with respect to COD fractions COD tot , COD ss , COD col and COD sol were 537.2 (165.3), 189.9 (109.5), 127.4 (75) and 223.4 (108.8) respectively. The investigations were conducted over a period of 630 days. The overall removal efficiency obtained on the basis of the distinguished COD fractions was far better than for the individual reactors, i.e efficiencies for COD tot , COD ss , COD col and COD sol , were respectively 68.7 (16.7), 51.2 (41.8), 62.1(38.2), and 71.8 (30.5) %. The imposed overall HRT was 7.4 (1.6) (i.e. 5 + 2.4) hours. The advantages of the two-step UASB reactor configuration include 1) the distinct higher overall removal efficiency of the anaerobic pre-treatment system 2) the higher sludge age 3) the higher reliability of the anaerobic pre-treatment process 4) the two reactors can separately be operated in case of technical problems.

Mots clés : waste water treatment - anaerobic treatment - feasibility studies - tropics

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Page publiée le 15 septembre 2007, mise à jour le 14 janvier 2018