David Stagg from Graf UK explains why innovative sequencing technology is the way forward for efficient wastewater management in developments
Nutrient neutrality that requires reductions in nitrates and phosphates, reducing ammonia levels in the effluent and carbon dosing – the world of wastewater treatment has become a dynamic, complex, and challenging one.
No longer is it just a choice between cesspool, septic tank, or wastewater treatment plant. Now there are options for wastewater treatment plants too – between conventional flow/activated sludge systems and those that employ advanced sequencing batch reactor (SBR) technology.
One of the main differences between these systems is that SBR combines several processes during the treatment cycle in a single treatment zone, in a chronological sequence which means a clarifier/secondary chamber is no longer required.
SBR systems run in batch cycles which generally operate twice or four times every 24 hours to ensure the treatment process is identical every time to produce high-quality effluent.
Continuous flow systems operate on a “what goes in, comes out” basis, so during periods of heavy loading/inflow the effluent travels through the system much quicker, potentially reducing the effluent quality.
The technology around SBR was implemented decades ago in Scandinavia, where reducing and monitoring phosphate and nitrate levels is common practice. More than 5,000 wastewater treatment systems with this technology have already been supplied throughout Europe to meet these challenging discharge constraints, achieving an independently tested 0.4 mg/l of phosphate removal with dosing and 1.6 mg/l without dosing for domestic systems up to 50 populations. Denitrification is used to reduce nitrates, achieving 7.9 mg/l. Larger population requirements would be designed individually.
The system of choice
The SBR process is a modern method for cleaning wastewater. Due to its many advantages – its operational flexibility and excellent process control possibilities – combined with the need to meet the increasingly stringent environmental discharge standards, it is becoming the system of choice for UK designers/specifiers and consultants.
Bespoke designs utilising advanced SBR technology are allowing these influencers to meet requirements for decreasing discharge levels of phosphate, nitrate, ammonia, biological oxygen demand (BOD) and suspended solids (SS). They also bring potential design solutions for more challenging applications such as breweries, distilleries, dog kennels, dairies, and chicken farms.
SBR technology can be employed in recycled and recyclable plastic or precast concrete tanks, with the former being particularly eco-friendly and designed for installation using granular backfill, which is not only more cost-effective than concrete but also quicker to install.
A SBR system is operated by air power generated by a blower/compressor, which means there are no moving parts or electrical components inside the tank itself. This reduces maintenance and replacement costs and makes for a more efficient operation.
Depending on the treatment objective, the system can control oxygen levels entering the treatment zone allowing nitrification and denitrification treatment processes, as well as varying the duration, frequency, and arrangement of the cycle phases. The plant can therefore easily be customised to meet individual project requirements.
Another benefit of this more advanced system is it comes with underload detection as standard, which allows it to operate efficiently with up to 75% ‘underload’ – a benefit particularly for second homes which are often left unoccupied for long periods of time. If the system is likely to drop below the 75% underload threshold, carbon dosing makes up the shortfall, as well as enhancing the treatment performance, to ensure low-odour and even odourless treatment and extended times between tank emptying.
SBR technology also differs from conventional systems in that no growth bodies are necessary, as the SBR process does not require the plastic media which can clog and escape from the tank, polluting the environment.
As an alternative to standard batch cycle timings, SBR systems can also be operated depending on the fill level, according to the inflow. If the system’s “buffer” recognises that only a small amount of wastewater has entered, it can automatically switch to economy mode.
Staying in control
An SBR system gives much greater control over the treatment process, which can be monitored and adjusted when needed to ensure optimum treatment levels. While it avoids the need for traditional tertiary/secondary treatment (such as reed beds which may become less effective over time and during the colder winter months) advanced tertiary treatment stages, such as UV, chlorination, and specialist sand filtration processes, are possible with the latest SBR technology.
Larger systems can be designed using multiple lines which can be combined and/or run independently, allowing 50% (two line system) or 33% (three line system) use of overall capacity when reduced loading (inflow) dictates. This style of design is ideal for holiday parks and camping/caravan sites which experience larger seasonal fluctuations in loading.
In a similar vein, a modular design principle can be used to future-proof longer-term projects where treatment plant capacity will need to increase over several years. Later phases can be added when the project growth dictates, and more capital expenditure is available.
Whatever the treatment processes, it is essential the system is commissioned by trained specialists who should also be responsible for its long-term maintenance and servicing.
David Stagg is technical product specialist for Graf UK