Author: Cheung, Kwok-yu
Title: Review of unit generation rates for wastewater flows and loads
Year: 1999
Subject: Sewage disposal -- China -- Hong Kong
Sewage -- Environmental aspects -- China -- Hong Kong
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Department of Civil and Structural Engineering
Pages: vi, 60 leaves ; 30 cm
Language: English
Abstract: In designing the capacity of a sewerage system, it is important to quantify the wastewater arising within the area where the sewerage system is to serve. In cases where the actual flows and loads are not known, particularly for future scenarios, it is quite common to derive calculated flows and loads. Flows and loads from domestic sources are usually calculated by taking the population figures for the area and multiplying them by the "per capita" daily generation rates. Where the wastewater is generated from an industrial area a "per capita" or "per hectare" flow is often assumed. The DSD Sewerage Manual (May 1995) is considered as one of the most updated sources of reference for deriving calculated flows and loads of domestic and trade effluent in Hong Kong. The unit load generation factors for domestic activities in the Sewerage Manual have taken into account the diurnal variation in transient population, particularly those working, or studying outside their home catchment. It had been assumed that a student or a "economically active" person would only discharge about 50% of their pollution load at home. The remaining portion would be discharged away from home at school or at work. Thus, the domestic load factors were separately derived for residential population and transient population. Based on the ratio of residential to transient population in 1989, the residential factors in the Sewerage Manual were adjusted to be 20% greater than the transient factor, as they are influenced by home makers and retired or unemployed people, who spend more than 50% of their waking hours at home. The acceptability of using these domestic factors was justified under this study by using the year 1997 population data. The results showed that these load factors are acceptable for use to quantify the pollution loads for existing situation. However, for estimation of future domestic loading, it is recommended to conduct the same justification exercise as carried out for this study and make necessary adjustments to the domestic load factors in the Sewerage Manual. On the other hand, the load factors for trade effluent provided in the Sewerage Manual might over-estimate the actual pollution load discharged from industrial and commercial premises for future scenarios. The reason is that these load factors were derived in 1989 for pollution at source. In calculating the actual loads received downstream of industrial and commercial areas in the future, allowance should now be made for the amount of wastewater pre-treatment facilities which is operating inside individual industrial and commercial premises. Based on the findings mentioned above, an improved model was developed in this study to quantify the pollution load generated in a sewerage catchment. A spreadsheet was also created to calculate the running cost for collection, treatment and disposal of wastewater discharged from a sewerage catchment. This spreadsheet was developed by linking the unit generation factors in the DSD Sewerage Manual to the Sewage Charge and Trade Effluent Surcharge rates recently derived by the government for the "Polluter Pays" Scheme.
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