Data’s written in the stars

Looking at the industry statistics, it seems that the water industry in the UK currently consumes something like 3% of all the electricity supplied to the grid. That means it is responsible for about 5M tonnes a year of carbon dioxide emissions.

In wastewater treatment at least 60% of the energy is used for process aeration alone. It follows that improving the operation of our wastewater treatment plants can make a significant impact on our carbon footprint and make the whole process more sustainable.

The problem is, of course, that the quality of raw sewage, measured by a large number of parameters, is variable, and that means that optimum process conditions keep changing – not easy for automatic control systems. We know that if we could have a dedicated process specialist on site 24/7 we could keep the process optimised, but that is just not a practicable option. Or is it?

Veolia Water Solutions & Technologies (WS&T) has developed a new version of its STAR (superior tuning and reporting) process control system. STAR2 combines feed forward and feedback control to optimise the process continuously.

Data from the raw sewage and in-process instrumentation – COD, ammonia, nitrate, phosphorus, pH, temperature, dissolved oxygen, for example – are fed into a sophisticated activated sludge process model that, in real time, calculates the desired values of process control parameters like sludge recycling flows and aeration rates. These are then used as set points for the process control instruments and controlled by feedback loops.

The difference between STAR2 and conventional SCADA-based distributed control systems is that STAR2 is holistic; it takes the entire process into account rather than simply regulating independent process units. For example, if it is a BNR (biological nutrient removal) plant, STAR2 calculates the best operating conditions for the whole consortium of bacteria.

Flow distribution

STAR2 works for any size and complexity of plant and encompasses a wide range of control strategies. In nutrient removal it optimises nitrification-denitrification phase control, dissolved oxygen, recirculation flow and nutrient carbon dosing and phosphorus precipitation.

In hydraulics it can be used to optimise flow distribution between plant sections on the basis of load variations, return sludge flow and stormwater control linked to a rain gauge to provide an early warning of high inlet flow and prepare the plant to accept it. In addition to reducing operating costs, improved process performance often means that the works can meet more stringent effluent requirements than those for which it was designed, or cope with an increase in hydraulic throughput without sacrificing effluent quality.

So how does it really perform live?

Engineers did a lot of the development work on the first version of STAR at a sewage treatment plant at Aalborg in Denmark. Over a ten-year period using the controller they doubled their water throughput and increased the nutrient removal without increasing their energy consumption.

Other trials in France, Belgium, Poland, Germany and Korea have shown a 20% reduction in aeration energy and a corresponding reduction in greenhouse gas emissions. But it is not just in energy reduction that STAR2 can improve wastewater treatment plant performance.

Evidence from operating sites has shown improvements in effluent quality by up to 30% in both nitrogen and phosphorus, a reduction of 50% in phosphorus precipitation chemical and reduced sludge production.

Veolia WS&T has developed two versions of STAR2:

· STAR2 Standard is suitable for small to medium-sized works up to about 40,000 population equivalent, using a single process like, for example, aeration ditches, trickling filters or activated sludge plants with or without denitrification. In this version, the system control modules are generic for the particular process being used.

· STAR2 Professional is for larger and more complex plants. Its control modules are custom-designed to meet the needs of plants where processes are combined together. This maybe, for example, where a biological nutrient removal plant has been installed or where tertiary treatment has been added. It might also be where an old biological filtration plant has been uprated by adding a parallel activated sludge plant, where there is unequal flow distribution or varying hydraulics between process lines and so on. In these systems the modules are integrated to provide a true plant-wide control and optimisation system.

So it is like having a process consultant on site 24/7, albeit a virtual one, and the advantages to be gained from improved process performance are tangible.

They include lower operating costs, increased plant capacity, more security in meeting consent limits and a smaller carbon footprint. n

Kieran Healey is synergies & integration manager at Veolia WS&T. www.veoliawaterst.co.uk