Nereda plus tertiary filtration: An alternative to MBR?

Choosing the right technology for your wastewater treatment plant is an important capital investment decision. For example, if you have an existing sludge reactor, should you replace it with a small-footprint, single-tank option such as Nereda or augment it with a membrane reactor?

Given the long lifespan of wastewater treatment plant (WWTP) assets, it may take years for the drawbacks of your chosen solution to become apparent. So, when comparing treatment technologies, you need to consider the next 20 or 30 years of operations (i.e life cycle cost analysis) as well as how your plant can achieve the performance it needs to deliver today.

You also need up-to-date, real-world information to make the most informed decision. And that can only come from a re-examination of all available technologies.

Membrane bioreactors: Reevaluating a popular treatment technology

Over the past three decades, Membrane Bioreactors (MBRs) have been used to augment existing CAS systems – and for good reasons.

MBRs deliver effluent water quality suitable for reuse in agriculture, industry, and household applications, making them an attractive option in the Middle East and other water-scarce areas worldwide. The membrane unit’s compact size means it delivers excellent quality without adding to the plant’s physical footprint – especially when immersed in the bioreactor. And, if the membrane is in a separate tank, sludge characteristics should have less effect on plant performance, making MBRs easier to operate than a conventional sludge system.

Or at least, that was the theory. While the quality of the effluent is undeniable, several limitations of MBRs have become clear as we learn more about what it takes to deliver that performance over the long term.

Pushing the limits of wastewater treatment sustainability

One key attraction of MBRs is that – whatever the influent quality – the membrane will constantly deliver consistent, high-quality effluent.

But because membranes are ‘always-on’, this doesn’t give WWTPs the flexibility to reduce treatment (bypass membrane filtration) if discharge permits allow or if treated effluent is not required for reuse application

As plants face high operating costs, is this over-engineered performance sustainable? Especially as MBRs, which are known for their highest-in-class OPEX, also require a series of energy-intensive steps in the process that precede membrane filtration.

While MBRs have become more efficient, these processes inevitably add to the energy consumption.

To keep energy costs down – and to protect the membrane – it’s often necessary to operate the biological process at a reduced biomass loading. Maintaining an optimum sludge level adds operational complexity and increases the labour costs required to manage it.

And the membranes in a MBR process have a finite lifespan. Even with careful, conservative usage, membranes must be replaced every 5–8 years, which adds significantly to a WWTP’s operating costs

Despite these challenges, MBR remains a viable treatment option, given its small footprint and excellent effluent quality. However, newer technologies have become established and offer similar effluent quality with more sustainable, cost-effective treatment processes.

Nereda + tertiary filtration: A flexible, modular alternative to MBRs

With over 100 plants worldwide, Nereda is proven to be an extremely efficient biological treatment to meet challenging effluent quality targets.

Combining aerobic and anoxic environments in one tank means Nereda requires only a quarter of the space of a CAS system – giving it roughly the same physical footprint as an MBR. But because Nereda uses less equipment, its energy costs are far lower.

The growth of the Nereda sludge is robust and can be easily controlled, making it operationally straightforward. And the fast-settling, granular nature of the sludge minimises the amount of solids that reach the filter.

The effluent quality from a Nereda treatment is often high enough to meet stringent nutrient levels for discharge to surface water environment without the need of tertiary treatment.

Nevertheless, Nereda can be easily augmented by adding a final filtration step including the flexibility on the type of tertiary filtration. Depending on the final effluent requirement, the choices of tertiary filtration can vary from a simple cloth filter or sand filter to membrane ultrafiltration. The combination of Nereda plus tertiary filtration can deliver effluent of a quality suitable for water reuse and equivalent to that from an MBR but at a significantly lower operating cost.

How one WwTP found the right treatment technology

A WwTP in the Middle East needed to expand capacity and improve the capability to deliver high-quality effluent for water reuse. However, the site had limited physical space, making MBR and Nereda the most suitable options.

To help our client choose between the two technologies, we compared the lifecycle costs of the two systems and operational considerations such as chemical usage and other ways to ensure effluent quality.

We found that the combination of Nereda with an ultrafiltration treatment significantly outperforms the equivalent MBR on energy consumption, operating expenses, and ongoing maintenance demands.

Our evaluation helped this client see the true lifecycle cost of both systems and select the best treatment technology with confidence.

MBR vs. Nereda: Five essential considerations

Every WWTP has unique requirements due to population, influent quality, regulations, environmental sensitivity, and more. This can add complexity when you need to decide on upgrades or expansions.

If you’re choosing between MBR and a technology such as Nereda that produces similar effluent quality, we recommend the following comparisons:

• Whole-life cycle costs including replacement cost and routine maintenance requirements;
• Quality requirements for the reuse application;
• Flexibility in operating the reuse portion of the plant at reduced capacity to match demand;
• Ease of operation at variable influent loads and effluent demands;
• Sustainability of the solution including required chemical and energy consumption.

While MBR consistently delivers fixed effluent quality, Nereda plus tertiary filtration can give your plant greater flexibility to respond quickly and effectively to your circumstances – i.e. flexibility to produce reuse water via tertiary filtration and flexibility to bypass tertiary filtration as and when needed dependent on the treated effluent needs, also the flexibility to accommodate higher peak flow during rain weather flow condition without the need to operate tertiary filtration, OPEX can be flexible depending on the needs or requirement.