The flow distribution in conventional settling tanks is often ineffective and solids are not well flocculated, limiting hydraulic capacity and reducing the settling efficiency of the tank. Retrofit solutions are available, but how can they best be used to enhance solids removal and unlock the spare hydraulic capacity that is available from conventional settling tanks?
Settling tanks do not dissipate the hydraulic energy of the incoming flow efficiently; the resulting currents in the tank can disturb the sludge blanket and lead to the occurrence of short-circuiting which impairs settling efficiency. Limited flocculation takes place, further reducing solids removal efficiency. Passive retrofit techniques can enhance settling tank efficiency but often lack a published design rationale to quantify performance improvements.
This project will:
- assess the extent of hydraulic inefficiencies and potential for improvement to residence time distribution by settling tank type, treatment process stage, and size of wastewater treatment works.
- provide simple relationships to assess how retrofits can be best used to enhance solids removal efficiency and unlock the spare hydraulic capacity
BUSINESS BENEFITS TO CLIENTS
- Improved compliance with effluent quality requirements.
- Less load on secondary treatment with savings in energy use and opex spend.
- Increased primary sludge production to enhance biogas, energy generation with resultant reduction of opex spend
- Avoided or delayed capex to handle increasing hydraulic loads (population growth/compliance) through retrofits to existing tanks
- Identify the main issues relating to the hydraulic inefficiency of settling tanks.
- Review the techniques available for improving the hydraulic efficiency of settling tanks.
- Evaluate the efficacy of the different techniques on site and develop a decision support tool.
- Provide design guidelines for improving the hydraulic efficiency of settling tanks.
PROJECT OUTPUT AND IMPLEMENTATION
- Database giving evidence on the performance of measures to improve the hydraulics of settling tanks including energy dissipation inlet to reduce the disturbance to the sludge blanket and to enhance flocculation, and the use of baffles to prevent the onset of short-circuiting.
- A simplified design procedure, including a decision support tool, for water company engineers and their consultants, that combines residence time flow distribution with surface loading rate (upward flow velocity) and hydraulic retention time to optimise settling tank performance and reduce reliance on costly computational fluid dynamics software.
RELATED WRc WORK
- CP440 Real time control - holistic management of wastewater, 2012
- CP195 Using sustainable technology to upgrade small wastewater treatment works, 2004