Investigation of control and on-line optimisation opportunities of a waste-water treatment plant

Northumbrian Water (NW) anaerobically digests up to 40,000 tonnes of sewage sludge (dry solids) and processes up to 12,000 L/s of raw sewage at its Howdon site on Tyneside. All sludge arising from NW's over 400 wastewater treatment works are either processed at Howdon or at the similar plant on Teesside. Howdon is critical to the company's (and the North East's) wastewater operations.

The Advanced Anaerobic Digestion (AAD) facility at Howdon was designed to achieve higher biogas (combination of biomethane and CO2) volumes by pre-treating the sludge in a thermal hydrolysis plant (CAMBI). The CAMBI plant consumes energy in the form of steam in order to further break down the composition of feedstock available to the AD process, but in doing so producing more net energy (i.e. producing more biogas) and vastly reducing retention times of AD digesters. A later addition added a biogas 'up-grading' plant where CO2 is removed from the biogas via a stripping column and its quality (calorific value) is raised such that it is suitable for injection into the gas grid.

NW must manage this plant so that it meets its two primary objectives: that of processing all necessary sludges so that they are capable of being re-cycled to land, and to optimise the revenues accruing to the plant whilst minimising the operating costs. In achieving this NW must be able to schedule planned maintenance, accommodate unforeseen breakdown, work with the constraints of the capacity of the local gas grid, decide whether to buy its own gas back in order to generate electricity for the site and generate and make appropriate use of the waste-heat coming from its combined heat and power plant.

Day to day operation of the plant is fully monitored, but its control relies on the experience, availability and knowledge of the operators. A supervisory control system taking current data input and validating operations choices would help in securing sludge processes, optimising net revenues, allowing greater use of the operators time are the company's other facilities, and provide assurance to the leadership regarding plant operation.

The aim of this work being to investigate and develop operational strategies (process control and optimisation) in order to improve process understanding, operation and site robustness; the project aims to deliver a technological solution to the operation and control of wastewater treatment site, focussing on the operational challenges at the Howdon site. This is an area that has not been considered in previously funded / collaborative work between Newcastle University and NW.
Initial investigations have developed a simple Mixed Integer Linear Programming (MILP) model of a small area of the site; work will be done to adapt the optimisation techniques employed to other areas of the Sludge Processing site, developing models that can be optimised in a similar scheduling fashion. An overall multi-objective retrospective analyser of how the site was operated and how it could have been operated will be created. Objectives for a multi-objective analyser could include (but are not limited to): optimising around Volume of Sludge processed, Maximising Profits, minimising waste.

Research into the relationships between volume of sludge processed and gas yield will be undertaken, which will ultimately be used within a whole site optimiser to be incorporated into an on-line supervisory control scheme. The on-line supervisory control scheme may or may not directly control the site, but it is the aim that it will use live operational data to provide accurate and up to date advice on the operability of the site.