Southern Water cuts carbon emissions by 25% with real-time control system

London - Southern Water is introducing a new advanced control system for six of its wastewater treatment plants in the Chichester area - one of the first such applications of real-time control in the UK. The project, which is being carried out in collaboration with MWH and its joint venture organisation, 4 Delivery (4D), is to help Southern Water to meet its AMP 4 wastewater standards.

With traditional control systems, it is not possible to make use of the safety factors built into a treatment works to take account of the wastewater variability and seasonal changes, according to Ajay Nair, MWH principal process engineer seconded to the 4D -  a JV between Costain, United Utilities and MWH, established to deliver over £700 million of Southern Waters’ fourth Capital Maintenance Programme 2005-10. 

Under AMP4, Southern Water had to provide total nitrogen removal at six of its treatment plants - Budds Farm, Peel Common, Chichester, Sidlesham, Thornham and Bosham. The first three plants use energy-intensive activated sludge processes and combining these processes with total nitrogen removal would have doubled energy use and increased carbon emissions due to the need for methanol dosing.

Southern Water and 4D decided to reduce the carbon footprint associated with each of these new plants by using real-time control. These will be amongst the first systems of this kind to use real-time control in the UK.

Activated sludge plants require oxygen for the various organisms involved to convert the incoming wastewater to a less reactive form. Oxygen is supplied by bubbling air into the reactor using blowers and diffusers. The amount of energy used in directly related to the amount of air supplied, so it is vital to make this part of the process as efficient as possible.

Traditionally, the amount of air supplied has been controlled using a direct measure of the oxygen concentration at different points in the reactor. Whilst this has been successful, the limitations associated with this method are numerous, so any additional improvements in energy reduction are limited.

"By controlling a treatment plant to match the actual conditions experienced at that time, rather than an “average” or minimum condition, optimum treatment and energy consumption can be provided. The term “real time” control has been used to describe this particular method of aeration control," explained Nair.

Southern Water said it now anticipates that carbon emission savings that can be achieved on all three of the schemes will be around 25%. It also noted the potential cost benefits of the energy and methanol savings from the use of real-time control systems.

Currently, the partnership has selected a preferred supplier and a preferred method of the advanced control system to minimise capital expenditure and maximise performance. However, Nair said that although, financial drivers are still very relevant, the sense of urgency around climate change is such that a greater emphasis is being placed upon carbon reduction techniques as the primary filter.

"Both Southern Water and 4D have demonstrated their commitment to pioneering technologies that actively reduce carbon emissions to meet the compliance standards applied," added Narinder Sunner, MWH principal process engineer.  "The work conducted to date sets the foundations for future energy reduction strategies not only for Southern Water, but for all UK water utilities if our national target of a 26 - 32% reduction in CO2 by 2020 is to be realistically achieved."

4D and Southern Water summarise the benefits provided by the real time control system as including:

· Rather than using oxygen concentration as the main control, ammonia is used to predict the amount of air needed so the system reacts quicker.
· By using ammonia as a direct measure, over-treating and wasting energy is avoided.
· The internal re-cycles can be optimised so only what is needed is pumped rather than over-pump.
· Seasonal conditions can be matched to take advantage of the faster reactions that occur in summer time reducing the energy wasted which would be used simply to treat by-products.
· The amount of methanol used by optimising the 2nd stage removal and reducing the impact of dissolved oxygen on consumption.