How often will sewage spill? [Stochastic analysis & design of a waste-water system with large RDII components]

For systems with large RDII (Rain Derived Infiltration and Inflow), classical methodology for estimating loading on Waste-Water systems can be dangerous if it portrays the impression of a known ultimate design flow. The reality, for such systems, is that ultimate loads cannot be determined. The paper presents real-world modelling to enhance planning and environmental aspects, of a sewer-system backbone for a “wet” area [1500 mm/year rainfall located at the base of steep sided valleys] by predicting the stochastic nature of outflows and overflows. Described is:

1. The use of RDII analysis to predict loading probabilities by;

a. Analyzing flow data to identify dry-weather loads then subtract same to determine wetweather components. (The latter “left-overs” proved to be the “main-course” and represent up to 90% of the load during times of high rainfall!)

b. Formulating a mathematical relationship between rainfall and RDII then use same to estimate loadings for various return periods.

2. Design & prediction of overall discharge frequencies and characteristics; by applying the load predictions, and modelling various scenarios, to ensure that, on average;

a. For annual events; all WW is treated and zero overflows occur

b. Every 2 yrs, excess flows receive simple treatment in a bypass system (but are still discharged through the normal diffusers into tidal flushed receiving waters)

c. Every 5 yrs, some of the bypass treated effluent is discharge to flooded turbulent streams

d. Every 10yrs, some untreated effluent is overflowed but only to flooded turbulent streams