A sewage treatment works (STW) with 4 single-stage trickling filters has become organically overloaded due to the discharge of a biodegradable industrial effluent to the STW. This is causing effluent to pond on the surface of the filters.
Below are data for the STW, showing the original design values for the trickling filters and their current operating values.
Original design values Current values
Dry weather flow (m3/d) 1,600 2,000
Influent BOD (mg/l) 200 350
Organic loading rate (OLR)
(kg /m3.d) 0.05 0.11
Hydraulic loading rate (HLR) (m3/m3.d) 0.25 0.31
Media type mineral mineral
Media Size 40 mm 40 mm
Media volume (total) 6,400 m3 6,400 m3
Number of filters 4 4
Filter type single stage single stage
Discharge consents 20/30 20/30/5
You have been asked by the STW Manager to compare 4 options to solve this problem:
(1) effluent recirculation; (2) double filtration; (3) alternating double filtration and (4) construction of an additional high-rate trickling filter.
Your solutions should make use as extensively as possible of the current trickling filter structures. You can change flow paths or packing media if required and build new structures if you feel this is essential to implement a desired solution.
Write a short report for the STW Manager including the following information:
An explanation of why the effluent ponding is occurring at this STW.
A comparison of the four design solutions, including the following:
• Schematic diagrams to show how each solution could work instead of 4 single stage
filters
• Calculation of new HLR and OLR values for each proposed solution – are these compatible with design guidelines?
• Comparison of proposed solutions in terms of the anticipated benefits for the STW
• Explanation of practical changes that will need to be made at the STW to implement each option
Analyse your results and make recommendations as to which solution (or even combinations of solutions) should be implemented.
Assignment Includes
Structure and presentation of short report
Explanation of methodology and approach and any assumptions made
Understanding of the implications of the different options being proposed
Clarity of discussion and motivation for the overall recommendation