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Why we love the Activated Sludge process

Ajay Nair

Technical director and an expert in water, wastewater treatment, and resource recovery. 

This year the Activated Sludge (AS) process will be 100 years old. This process will remain a cornerstone technology - and would form part of my ideal AS plant.

A Hypermixer in an Activated Sludge PlantA Hypermixer in an Activated Sludge Plant

It's official − water engineers love the AS process. With the exception of those who operate a poorly designed AS system, a quick survey confirms, we see it as an essential technology. It enables us to meet recent environmental legislation including tighter standards, that would be difficult to achieve using technologies like our other much loved process – the trickling filter.

Today, we have to consider tighter ammonia consents, disinfection to meet bathing standards, phosphorus removal and increasingly nitrogen removal. Key concerns include current and future power costs − 20 years ago power cost less than 5p per kilowatt hour (kWh) – today it can be 19p/kWh.

Therefore, to make best use of AS technology we must design for efficiency considering daily and seasonal tariffs and extremes of climatic conditions, especially as AS can use more power than less effective trickling filters.

To create a highly efficient AS plant for now and the future, despite many other interesting possibilities, my vote would go to a relatively traditional system, using technology available now, such as a nitrifying AS process using fine bubble diffused aeration (FBDA).

With FBDA, it’s essential all the design consequences on alpha factors are understood because they are heavily influenced by their operational environment. They measure how efficient a diffuser is and have a huge impact on overall plant efficiency. FBDA performance has improved significantly in recent years with today’s aeration systems often quoting a value of 8% per m – compared to the previous 5%.

However, while diffusers increase efficiency, an increase in back pressure can be required to overcome any extra resistance to achieve improved oxygen transfer. Similarly, recent studies show a marked drop-off in the first two years owing to the deterioration rate of performance in diffusers, both in terms of backpressure and transfer efficiency.

We now understand alpha is not constant during the day and can vary by as much as 100%, especially in the early stages of the AS reactor. Research is still limited but AS plants including a denitrification zone find it reduces fouling and increases alpha values.

This results in a denitrification credit for oxygen and alkalinity plus an alpha credit, further reducing energy consumption.

The penalty for including a denitrification zone for mixing and return pumping is minimal so inclusion can be justified by energy savings and improvements in effluent stability.

Mixing technologies have progressed significantly if installing anoxic zones for either selection or denitrification. Twenty years ago, recommended energy intensity for mixing was 10W/m3. With todays mixers running 24/7, even this modest consumption contributes significant values.

Fortunately, systems like the hyperclassic mixer can give large savings by reducing energy input to less than 1.5W/m3.

Alpha factor
As influent wastewater has high levels of variability, it is important the system can respond with the minimum of control error to maximise efficiency. By far the best control system for aeration is a most open valve system (MOV) with airflow control on each zone, cascaded to a dissolved oxygen (DO) set-point.

This system could be further improved by using a direct measure of the fouled alpha value to calculate an actual airflow set-point rather than relying on the DO set-point error.

Using off-gas measurements the alphameter is now commercially available and has demonstrated savings through improved control. This technology opens up exciting possibilities enabling us to understand how this parameter constantly changes, get insight into cleaning and replacement regimes and to improve design.

FBDA reactors are inherently inefficient as most of the oxygen demand is required at the front end and FBDA diffusers are most affected by wastewater conditions and have the lowest alpha factor at the very front of the AS reactor.

Historically, we have overdesigned AS reactors, so we know there is spare capacity. This is typified by practically zero ammonia levels, two thirds down the reactor.

Indicating the last third is simply mixing even though this is the most efficient part of the reactor. To operate with greater efficiency, we must adjust the ammonia flight path.

Using monitoring instruments and real-time control models, it is now possible to control demand in the reactor to balance airflow, make better use of the efficient parts of reactor and avoid wastage reaching minimum airflow for mixing. By implementing this in flight measurement approach, we have achieved significant savings even where highly sophisticated DO control systems are already in place and generated amazing improvements to the stability of final effluent performance.

This has encouraged us to take out unnecessary conservatism, reduce footprint and deal with the challenge of tightening consents.

Could environment reactive consents be round the corner? This could change effluent quality to reflect water course flow, meeting water quality and reducing carbon impacts simultaneously.

Diffuser needs
Significant results can be achieved with just a few strategically placed ammonia and DO probes. Examples of advanced real-time control have been operating successfully for more than three years within United Utilities, Southern Water and Wessex Water.

Making best use of reactor management systems means designing against convention. Traditional tapering of diffusers needs to be revisited to avoid losing savings because of the cost of higher airflow per diffuser. A move to a more uniform aeration grid down the reactor must be considered.

The number of control zones also needs careful consideration. There are moves towards two zones of control and reliance on tapering as the preferred method to minimise capital.
However, moving from two to three and then four can create significant energy savings and improve effluent stability.

As design considerations become more complex; to understand operational efficiency and compliance, we must expand the use of dynamic simulation tools. For instance coupling process modelling with mechanical system performance models gives us more than the average, minimum and maximum design points for blower sizing.

Blower technology has also moved on, with more efficient systems from APG-Neuros, Aerzan and Lontra. Using flight-mode to establish internally the best operating point to deliver the required airflow can bring massive savings benefits but will be obliterated if the system isn’t analysed and the right one installed.

What is crucial is designing for load and for tariff. Today’s process engineer must minimise power consumption during peak cost times while maintaining compliance. Adding infrastructure that allows extended go-slows is key in modern day plant.

Another improvement is running at the appropriate sludge age to reflect growth conditions. This reduces aeration demand propagated by bacterial endogenous respiration (the energy used to “eat the dead”) and provides more potential for gas production in anaerobic digestors.

My ideal plant would be an MLE (that is a big, fat anoxic zone) configuration utilising FBDA without a significant diffuser taper. The airflow demand envelope would be calculated with an MOV airflow control delivery system incorporating an off-gas airflow set-point.

I would have feed forward ammonia control with three if not four, control zones with an integrated blower management system. Depth would be driven by available footprint, but calculated by site performance and blower type. Blower arrangement would be selected by system modelling based upon my process modelled airflow envelopes – likely to be of multiple units to give turn-down flexibility.

I would also have sludge age control to keep operating at the biological sweet spot. Last but not least I would have used a process model to refine the design based upon a well characterised influent.

I believe, the AS process has many years left as the cornerstone in of treatment plants. With continual evolution, I’m looking forward to what the next 20 years will bring but also hoping for a revolution in the way we treat wastewater. 

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