How to make wastewater treatment better for the environment

Sewage treatment ponds on the outskirts of Blenheim in the Nelson region. Image credit: iStock

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Wastewater treatment is an essential but unseen urban service, often one we take for granted, unless something goes wrong.

Yet wastewater treatment also has unseen environmental impacts, contributing to greenhouse gas (GHG) emissions and the acidification of the ocean. As the urban footprint grows, so too will the environmental harm footprint, and the compliance costs associated with trying to rectify it.

At Aspiring Materials, we’ve demonstrated how magnesium hydroxide can capture and mineralise carbon dioxide emissions safely and permanently. With that science in hand, our magnesium hydroxide could now support wastewater treatment plants (WWTPs) to reduce their overall GHG emissions and help counter ocean acidification, all while also gaining operational efficiencies.

Sound too good to be true? Walk through this one with us…it may surprise you how possible it is.

Tallying up the environmental task

Wastewater treatment plants are a source of greenhouse gas emissions. Collectively, they contribute approximately 1.6% of annual global greenhouse gases, on par with emissions generated by the aviation industry.

At a global level that may seem minor compared to heavy industries like cement and steel. But at a regional level, wastewater sector emissions can account for up to 90% of the direct GHG emissions inventory that a city manages. As populations grow and urban environments expand, the emissions impact will also increase.

Methane (CH4) and nitrous oxide (N2O) have a 27 and 273 respectively greater warming potential than carbon dioxide (CO2). CH4 lasts significantly shorter than CO2 but still poses a greater warming risk. Source: IPCC Sixth Assessment Report (AR6).

Wastewater GHG emissions comprise the big three: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O).

The majority of these GHGs are methane and nitrous oxide (~70%). Unfortunately, these are the hardest emissions to prevent, yet they pose a greater risk to atmospheric warming: methane has a 27 times greater warming effect than CO₂, nitrous oxide a massive 273 times greater.

If a wastewater treatment plant is powered by renewable energy or biogas, then the CO₂ emissions only come from the breakdown of organic matter. That CO₂ is defined as biogenic, meaning it is part of Earth’s present day carbon cycle.

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Biogenic and non-biogenic CO2 emissions – what’s the difference?

Carbon dioxide (CO₂) emissions can be categorised by how they’re produced.

Biogenic CO₂ is part of Earth’s natural carbon cycle where CO₂ in the atmosphere is absorbed by plants during photosynthesis and then released again when those plants die and decompose. Biogenic CO₂ emissions are considered ‘carbon-neutral’ because the release of CO₂ is generally offset by a re-absorption that occurs in a short timeframe or ‘fast’ carbon cycle.

Non-biogenic CO₂ emissions come from the combustion of fossil fuels like coal, oil and natural gas. These sources of CO₂ have been stored underground for millions of years, so when extracted and burned it adds more CO₂ to the atmosphere than the Earth can balance out via the fast carbon cycle.

In carbon accounting, or a lifecycle analysis (LCA) that assess the environmental impact of a product or service, biogenic and non-biogenic CO₂ should be reported separately as each category has a different impact on the overall emissions.

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Biogenic CO₂ emissions, including those from wastewater treatment, are not counted in national emissions totals, because CO₂ released from organic matter was recently removed by plants through photosynthesis.

Emissions are not the only environmental impact of wastewater treatment though. As treated wastewater flows out to rivers and oceans, it introduces acidity into the water, which contributes to the larger problem of ocean acidification.

Marine ecosystems depend upon a stable environment where pH of the seawater is slightly alkaline. As the ocean absorbs more CO₂ from the atmosphere, the pH decreases to become more acidic and this impacts marine life. Image credit: Giles Charvet, ArtGrid.

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The imbalance of pH in the water is hazardous for aquatic and marine life and negatively impacts the ocean’s ability to act as a carbon sink that draws down CO₂ from the atmosphere.

Managing these environmental factors attract costs as WWTPs must meet local and regional regulations, so cost-effective reduction strategies are critical for wastewater treatment.

The most cost-effective way to reduce overall wastewater GHG emissions

Like many industries, wastewater treatment plants are being challenged by central and local government to report and address GHG emissions generated directly by their activities (Scope 1 emissions).

These plants receive wastewater material from a wide catchment area, often comprising both residential and commercial areas. That makes it difficult to predict what is in the waste material, so preventing the release of potent greenhouse gases can be challenging.

Technologies are in development, but right now, one of the most practical options for wastewater treatment plants to reduce their GHG emissions overall could be to use carbon dioxide removal (CDR) methods.

Despite wastewater CO₂ being counted as biogenic (effectively zero), if it is permanently removed, that removal can be counted as negative emissions. That negative balance can then be applied against CH₄ and N₂O emission totals, effectively reducing the total reportable emissions.

That makes permanent CDR key to achieving net-negative emissions.

Broadly, CDR is a suite of technologies and methods that takes CO₂ out of the atmosphere and stores it permanently. Wastewater treatment plants can take advantage of CDR and count CO₂ removal against the overall GHG emissions total. But the CDR must happen on site and be quantifiable.

This is where Aspiring Materials’ magnesium hydroxide can help.

Magnesium hydroxide: powering operational gain and emissions reduction

At Aspiring Materials, we’ve proven the carbon capture power of magnesium hydroxide.

By simply adding water to magnesium hydroxide and exposing that mixture to CO₂, an almost instantaneous reaction occurs that transforms the CO₂ into a safe, stable, solid material called magnesium carbonate. This effect is commonly referred to as ‘mineralisation’.

This science can be applied to wastewater treatment in two distinct ways that also bring additional benefits to the treatment process.

Application 1: Alkalinity dosing to correct acidity and remove CO₂

Wastewater is inherently acidic and unless corrected, the acidity has a negative impact on waterways and oceans. Acidity causes a pH imbalance that is detrimental to aquatic and marine life and contributes to the broader issue of ocean acidification.

To counter this acidity, wastewater is dosed with alkaline minerals to restore pH to a neutral or slightly alkaline range. Commonly used alkaline minerals are quicklime (calcium oxide), caustic soda (sodium hydroxide) or milk of magnesia (magnesium hydroxide).

Magnesium hydroxide offers an operational advantage over the first two because:

• it’s safer to handle, which means safer workplaces

• it offers higher alkalinity per kilogram, so less is needed to achieve the desired result

The added advantage, from an emissions standpoint, is that magnesium hydroxide (Mg(OH)₂) also mineralises the dissolved carbon dioxide in wastewater that is created by microbes breaking down organic matter.

That means less ‘off gassing’ or release of biogenic CO₂ during subsequent treatment phases.

And because mineralisation is a permanent removal it can be counted towards net emissions totals.

When magnesium hydroxide is reacted with carbon dioxide, magnesium carbonate is formed. This safe and stable method to store carbon dioxide is the same material climbers use as climbing chalk. Image credit: Cristian Rojas, ArtGrid.

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Once mineralised, the result is magnesium carbonate (MgCO₃), a safe, stable material—basically climbers’ chalk. It has uses in several industries including food manufacturing, construction, sporting equipment and paint.

Application 2: Biogas scrubbing to clean up biogas and capture CO₂

Gaining cost-efficiencies across public services like wastewater treatment is imperative. It’s a core reason why many wastewater treatment facilities capture their methane rather than flare it (burn it) and upgrade it to use as a biogas. While methane is a harmful gas that traps heat in the atmosphere, it is also a valuable heat and energy source when cleaned (or ‘scrubbed’) of other contaminants. The key contaminants include CO₂ and hydrogen sulphide (H₂S).

If a WWTP wants to generate energy from its biogas or refine it further to sell it as the more valuable biomethane, the H₂S needs to be removed as it causes corrosion and damage to turbine engines.

Methods to remove H₂S from biogas for energy use include chemical precipitation (to form solids that can be removed), using carbon filters (to trap H₂S), and chemical removal or “scrubbing” (using alkaline materials to absorb H₂S).

Magnesium hydroxide can play a role here too. It can scrub the biogas of H₂S and capture and mineralise CO₂ present in the biogas.

That carbon capture also counts as permanent carbon dioxide removal.

Biogas can be generated onsite at wastewater treatment plants from the methane released from the treatment process. Image credit: MXR Productions, ArtGrid.

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Combining methods for true net-negative emissions

In both solutions, CO₂ is mineralised and can be counted as permanent carbon dioxide removal.

Combined, the total amount of CO₂ removed via these two methods is greater than the amount of methane and nitrous oxide generated, achieving a net-negative balance.

When carbon dioxide is captured and removed from wastewater treatment, it can create a net-negative emissions profile for WWTPs. Source: Aspiring Materials

In the alkalinity dosing method, when the biogenic CO₂ in wastewater is mineralised, the magnesium carbonate can either be extracted and dried to on-sell to industries like the ones mentioned above or allowed to flow out to waterways and eventually the ocean.

Magnesium is already abundant in seawater and is a fast-dissolving but slow-release source of alkalinity.

So when wastewater that contains magnesium hydroxide or magnesium carbonate flows out into waterways and the ocean, it does not pose any risk to marine life.

In fact, the addition of magnesium also helps counter ocean acidification by increasing alkalinity. The mineralised carbon will remain permanently stored too.

That could be a huge win for climate.

Efficiencies beyond emissions

Reducing emissions is high on our list of must-dos, but we also understand how efficiency and economics have great bearing on which technologies are chosen to do the job.

Magnesium hydroxide offers a range of co-benefits for wastewater treatment that make it a tantalising choice for WWTPs.

Wastewater treatment plants can achieve additional benefits by using magnesium hydroxide as it:

• provides almost twice the alkalinity per kilogram than caustic soda or quicklime

• reduces odours caused by hydrogen sulphide

• limits corrosion in pipes

• improves the treatment process by removing water from wastewater sludge

• does not create toxic by-products when used and is safer to handle

• offers slow-release alkalinity, which helps control pH buffering

In short, magnesium hydroxide delivers benefits that any wastewater treatment plant can appreciate.

However, magnesium hydroxide has traditionally been more expensive than caustic soda and quicklime as it often needs to be imported from distant sources. Quality can also be inconsistent and contain impurities that need to be removed before use.

Plus, if we consider indirect emissions (Scope 3) from the mineral extraction and production processes, magnesium hydroxide is worse for the environment.

But with Aspiring Materials, we remove both the cost and emissions drawbacks.

Low-carbon, locally produced magnesium hydroxide

At Aspiring Materials, we produce clean critical minerals from an abundant source: olivine-rich ultramafic rock. One of our critical minerals is magnesium hydroxide, and due to our process, no emissions are released.

Conventional production of magnesium hydroxide requires high heat from fossil fuel sources and CO2 is also released from the calcination process. Aspiring Materials uses an electrochemical process and feedstock that has no carbon. Source: Aspiring Materials.

The result is a consistently high-purity, low-carbon product, every time.

Right now, we’re producing our magnesium hydroxide in Christchurch, New Zealand at volumes that allow for pilot trials with local wastewater treatment plants.

Being local is an added benefit, reducing transport miles from source to use and we’re closer to support trials and collaborate on specific requirements each wastewater treatment facility may have.

Our mission is to lower the barriers for industries like wastewater treatment to reduce their overall emissions in a cost-effective, efficient way.

Wastewater can be a net-negative emissions operation with Aspiring Materials

Wastewater is a true waste-to-value opportunity. Using magnesium hydroxide to increase alkalinity and clean biogas benefits both the wastewater treatment plant and the environment beyond.

Magnesium hydroxide can be used at key points across the entire wastewater treatment system that provides both operational benefits and a reduction of CO2 emissions. Image credit: Georgia Sigglekow for Aspiring Materials.

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With Aspiring Materials, wastewater treatment plants can have confidence in a low-carbon, consistent quality mineral that’s locally produced.

Wastewater treatment may be an unseen service to society, but by switching to Aspiring Materials’ magnesium hydroxide, wastewater treatment can be transformed into a climate-positive operation, reducing emissions, improving efficiencies and protecting marine ecosystems.

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This blog article is based on our technical paper ‘The case for magnesium hydroxide as a solution to wastewater sector emissions’, written by Dr Simon Reid, Chemical Process Engineer at Aspiring Materials.

Aspiring Materials has developed a method to extract magnesium hydroxide from olivine-rich silicate rocks. Our patented process produces no carbon dioxide emissions, and we regenerate our waste so it can be reused or safely returned to the environment.

When compared with lime, caustic soda or conventionally produced magnesium hydroxide, Aspiring Materials’ magnesium hydroxide provides wastewater treatment facilities with the lowest carbon footprint product available.

Contact us to find out how Aspiring Materials’ magnesium hydroxide can help your operation achieve an all-in-one solution for wastewater treatment.