The silent treatment: How minerals are giving water a new lease of life

Urban wastewater, commonly referred to as sewage, is generally a mixture of domestic waste from baths, sinks, washing machines and toilets, as described by the UK’s Department for Environment, Food & Rural Affairs (Defra).

The United Nations estimates that the amount of wastewater produced annually is around 1,500km³, six times more water than exists in all the rivers of the world. Every day, 2m tonnes of sewage, industrial and agricultural waste is discharged into the world’s waterways. Official statistics also show that 80% of sewage in developing countries is discharged untreated into waters, and a total of 70% of industrial wastes are disposed of in rivers.

In the UK, around 347,000km of sewers collect more than 11bn litres of waste and this is treated at about 9,000 sewage treatment works before the effluent is discharged to inland waters, rivers and the sea, according to Defra. In contrast, in the US, at least 1bn gallons (4.55bn litres) per day of treated wastewater is reclaimed to meet non-drinking water needs, such as irrigation of golf courses and public parks, according to the Center for Sustainable Systems at the University of Michigan.

If left untreated, the wastewater householders and businesses produce every day would damage the aquatic ecosystems and create public health problems. Untreated wastewater contains organic matter, bacteria and chemicals. According to Defra, if there were large or continuous untreated discharges of urban wastewater, the result could be too little oxygen for fish to survive in UK waterways. The purpose of wastewater treatment is to remove organic substances to protect the environment from these issues. This is where industrial minerals step in.

There are a variety of industrial minerals that are used in wastewater treatment – each yielding specific advantages. Minerals such as bentonite purify the water, while lime products have historically been used to neutralise potentially hazardous wastewater and, in recent years, magnesia has been used to neutrliase waste acid. 

The US uses around 566,000 tonnes of lime for its wastewater treatment systems. The above wastewater treatment plant is based in California, US.  (Source: Michael Layefsky)

Lime products

Lime products have been traditionally used to treat wastewater at municipal treatment plants to control the pH and alkalinity, precipitate nutrients and remove odours. 

To manufacture these products, quarried limestone is crushed and screened before being burnt in kilns to produce calcium oxide (CaO), also known as quicklime. Water can be added to quicklime to produce hydrated lime. Hydrated lime is added to sewage sludge at some municipal wastewater sites to kill pathogens, making the sludge suitable for recycling for use in agriculture. According to lime producer Lhoist, sewage water can be treated close to where it is created or collected and transported via a network of pipes and pump stations to a municipal plant. 

According to the US Geological Survey (USGS), the US used a total of 1.52m tonnes of lime in water treatment in 2013. Out of this figure, a total of 566,000 tonnes was used in wastewater treatments. A further 868,000 tonnes was used in drinking water and 87,000 tonnes used in acid-mine drainage.

The lime products used for these various applications differ from each other chemically and affect the treatment process in different ways.

Calcium hydroxide (Ca(OH)₂),  also known as hydrated lime, is used as a flocculent in the purification of wastewater, isolating and removing oils, heavy metals and suspended metals. It can remove metals from industrial wastewater. 

According to Finnish lime producer Nordkalk, hydrated lime is strongly caustic. With these products, the pH level of wastewater can easily be regulated to the desirable level. In contrast, calcium carbonate (caCO3) is slightly caustic and dissolves slowly. The product works best when ground finely and when the pH of the wastewater is less than 6.5, Nordalk says.

According to the USGS, lime also aids in clarifying and in destroying harmful bacteria. In its 2013 Minerals Yearbook, it states: "The leading use in sewage treatment is to stabilise the resulting sewage sludge. Sewage sludge stabilisation, also called biosolids stabilisation, reduces odours, pathogens and putrescbility of the solids." Lime stabilisation involves mixing quicklime with the sludge to raise the temperature and pH of the sludge to minimum levels for a specified period of time. 

Another benefit of using limestone in the wastewater treatment process is that supplies are globally abundant, according to Derek Thompson, product manager at lime producer Lhoist Northern Europe. The company’s UK arm mines lime in Derbyshire and converts it into high-end treatment products. 

Speaking to IM, Thompson said wastewater treatment companies have easy access to lime, as it does not have to be transported over long distances. He also said that lime products normally come in a "dry, dusty powder form" and can be used in large wastewater treatment processes.

Quicklime is similar to calcium carbonate, but is strongly caustic. It is predominately used in potable (drinking) water systems to either soften the water, remove hardness or adjust the pH. The World Health Organization (WHO) states that the pH of most drinking water lies between the 6.5-8.5pH range. 

US lime specialist Mississippi Lime Company’s sales director Robert Rasche says that demand for quicklime is growing. "Quicklime [is] historically, and continues to be, a very cost-effective method of potable water treatment," Rasche explains. 

This is a steady market, according to Rasche. "Growth rates in the use of quicklime in potable water are modest at 2-4% per year."

Trends supporting lime consumption in the water treatment industry include stricter environmental regulation in Europe and the US to protect both the water environment and drinking water. For instance, the European Union (EU) established the Urban Waste Water Treatment Directive in 1991 and the US implemented its Clean Water Act in 1972. 

Thompson says that new markets are also opening up in the UK because some London wastewater treatment companies are having to remove heavy sulphates from sewage systems. Lime can be used to remove the sulphate and neutralise the water acid. As a consequence, demand for lime products is starting to increase.  

Thompson says that he is noticing a new trend in the use of lime to treat the by-product of the biogas process – digestate sludge. Lime is used to sanitise the digestate and get rid of bacteria, such as salmonella, so that it can safely be used as fertiliser.

He explains that lime is still by far the most commonly used mineral to treat wastewater globally and its application in this industry can be dated back to Roman times.

However, using lime does come with challenges. Lime can be abrasive, hazardous and can generate a lot of sludge. There can be a functional disadvantage. As the material comes in a dry, dusty powder form, significant amounts can block water processing equipment. Furthermore, because workers have to work in a dusty atmosphere, they have to use respiratory masks to be able to breathe. In contrast, minerals that come in liquid form can be dosed through a dosing pump and used in small wastewater treatment plants. 

Lime is also used in numerous processes to treat mining-related discharges in active or abandoned mines, according to the USGS. The organisation says that these processes include the treatment of acid-mine drainage from operating and abandoned mines, specialised treatment processes such as catalysed cementation of arsenic and other heavy metals, and treatment of mine tailings that result from the recovery of precious metals to recover cyanides. 

The USGS also maintains that lime is used, generally in conjunction with soda ash (Na₂CO₃), for softening municipal and plant process water. This precipitation process removes soluble calcium and magnesium cations that contribute to the hardness of water, according to the organisation. This process also reduces carbonate alkalinity and total dissolved solids. 

In the US, lime consumption for drinking water treatment decreased in 2013 to 868,000 tonnes, down by about 6% compared with 2012’s usage figure of 920,000 tonnes.

Moorefield, West Virgina, US, wastewater treatment plant.  Chesapeake Bay Programme

Magnesia products

Magnesia products for wastewater treatment have gained prominence over the last three-to-four decades. Like lime and caustic soda, they neutralise wastewater. 

Magnesium hydroxide (Mg(OH)₂ or MDH) slurry is one of the key magnesia products used in wastewater treatment and is produced primarily by either calcining magnesite ore (MgCO₃) at temperatures of 700-1,000°C, which is then slaked to form MDH slurry. It is normally trucked from the site of production to the place where it will be used for water treatment. It typically comes in a solid form in a water solution. 

Magnesia experts concur that the mineral has two main advantages. First, it is easy and safe to handle. Second, it is insoluble. According to the USGS, in 2012, environmental applications (water treatment and stack-gas scrubbing) remained the largest tonnage end use for domestically produced caustic calcined magnesia (CCM), accounting for 43% of the total market.

Israel-based Dead Sea Periclase, a division of the Israel Chemicals Ltd Industrial Products (ICL-IP) group, is a well-known producer of high-quality magnesia. Roland Murenik, vice president of business development and research at Dead Sea Periclase, told IM that MDH is insoluble and used as a suspension. It almost works like the medicine, Milk of Magnesia, that is used to cure indigestion, says Murenik.  "It is like neutralising the acid in your stomach." 

Murenik says that, unlike caustic soda, which is very easy to overdose when neutralising wastewater treatments, you "cannot overshoot with the pH" with MDH.

"This is because if you overdose with MDH, you will meet the maximum pH, which is 10/10.5. In relation to caustic soda, it is easy to overdose to enormously high pHs of 13/14 – if there is an overdose for any reason."

"If the cost of caustic soda is around $400/tonne, there is an economic advantage to using MDH generally," he continues. "MDH for this application will typically sell for $500/tonne, but you use far less of this than you would with caustic soda."

Currently, caustic soda is priced around $400/tonne, but it varies by region and geographical market. Most countries have their own production of caustic soda. According to industrial chemicals specialist AkzoNobel, caustic soda is produced from the electrolysis of salt. "A concentrated solution of purified salt (NaCl) in demineralised water (i.e., brine) is decomposed in an electrolytic cell by the passage of an electrical current (DC)," the Dutch company outlines.

"The demand for caustic soda is not solely determined by the demand of caustic soda itself. Caustic soda is produced as a by-product of chlorine," Murenik says. "If there is vast demand for chlorine, then there will tend to be an excess supply of caustic soda in the market and price of caustic soda will drop." 

Dave Johnson of US-based Premier Magnesia concurs with Murenik that MDH has many benefits: "It produces less sludge at the back end of the wastewater treatment process. Therefore, you pay less in disposal costs. Certainly, MDH is safer than caustic soda," he says. "If you put your hand in caustic soda it will burn it. MDH is non-hazardous and safe to handle."

As a result, MDH provides a safer environment for wastewater treatment plants. "We [Premier Magnesia] have a high-quality MDH," Johnson told IM. "You can get low-quality ones, but they can end up clogging pipes and underperforming. They may work for a little while, but they will not work in the long term."

Environmental standards

Industry experts agree that magnesia is a bit of a new kid on the block in terms of using it to treat wastewater, compared to limestone which has been used for centuries. The US and Japan are two of the main users of magnesia globally, according to Murenik.

Japan-based Ube Material Industries is a magnesia producer. Ube Material’s director, Akio Ishida, told IM that environmental regulation is driving more companies to use magnesia for water treatment applications.

He says Japan revised an environmental standard in 1999, which affected the industry. The standard maintained that boron content in hot spring wastewater needed to be "tentatively under 500ppm and finally under 10ppm in the future". According to Ishida, boron is a naturally-derived ingredient in some hot spring waters and it is "quite difficult to control its content by using existing facilities".

Therefore, he says that new wastewater treatment systems have needed to be set up to conform to this environmental standard. This is where magnesia comes in. "We suppose that a chemical process utilising highly reactive magnesia for boron removal will be competitive against other systems like ion exchange membrane/resin, or others like activated carbon process – for example, for economic and handling reasons."

In Japan, mine wastewater often contains sulphuric acid since there are many volcanic stratums, Ishida says. He explains: "In many cases, before discharged into a public water area, [the acid] must be neutralised not to exceed the pH standard of wastewater."

Usually, cheaper limestone is used for neutralisation. Recently, however, in order to reduce insoluble by-products and to avoid secondary damages like sedimentation in dam lakes, partial use of MDH has been considered, Ishida says.

Nevertheless, even though using magnesia in water treatment yields a number of benefits over its competitor materials, there are still some problems preventing it from being used more widely. 

Murenik says that there are only a few MDH facilities in the world and supplies of the mineral are short in Europe and the UK, where usage is small.  Murenik points out another difficulty with using MDH. He says that it "requires a little bit more skill in installing the installation". 

"Once (…) installed, it is the same as if one is using caustic soda or lime – it needs to be installed by professionals."

Salt and polonite

Wessex Water, a UK-based water and sewage treatment company, does not use magnesia in its water treatment operations. However, it does use minerals such as hydrated lime and salt. 

Wessex Water uses salt in drinking water treatment in a number of ways. "The most familiar use will be the regeneration of ion exchange media using brine," a spokesperson for the company told IM. 

"Ion exchange media are used to provide softened water for chemical solution make up and in three nitrate removal plants in the Wessex Water region [south west England, UK]."

The company also has an on-site electrolytic chlorination plant. This creates sodium hyprochlorite from salt, which can then be used for oxidation or disinfection processes. Wessex Water is also looking to potentially use other minerals in the near future for its sewage treatment operations. 

The spokesperson told IM that the company is looking to trial a new technology next year called Biomag, which employs magnetite (Fe₃O₄) to enhance phosphorus removal.  He explains that Biomag uses ballast technology to increase the specific gravity (SG) of the biological floc in aeration plants [a facility which brings air and water into intimate contact, according to water specialist, GE Power]. 

Magnetite is a heavy material. When used in this process, it bonds with chemical pollutants and weighs them down. Magnetite has an SG of 5.2 and it also has a strong affinity to biological solids.

Wessex Water’s spokesperson explained that the addition of magnetite can increase the settling rate of the biological floc in aeration plants, and this means that the mixed liquor concentration can be increased, and can also result in improved solids, phosphorus and nitrogen levels or even a smaller settlement tank. Comag is a similar process using magnetite to enhance settlement after chemical flocculation, again potentially resulting in lower levels of phosphorus and metal residuals in the final effluent.

Competitor materials

Despite their widespread use, minerals do not have the water treatment market cornered. New research shows that the extracts from seeds of the Moringa Oleifera tree can be used for water purification. In a new study, researchers from Sweden-based Uppsala University claim that the Moringa seeds can also be used to separate minerals in the mining process.

Lead researcher on the study, Adrian Rennie, told IM that the seeds are used as a flocculating agent. The seeds contain a high fraction of protein. The protein binds to itself and binds to the surface of a lot of different materials (including impurities in the water).  Rennie says the seeds act like an "effective glue to stick particles together". This makes it easier for the human eye to see them and enhance removal.  

He said that there had been interest in the new technology in developing countries, partly because the seeds are readily available, but also because there is "very little risk associated with using too much of it".

"You may have heard of people adding synthetic flocculants to drinking water, where too much aluminium sulphate has been added to the drinking water supply by accident. Subsequently, they have to tell the public to stop drinking it for a while. In contrast, the seeds are edible. If somebody adds too much it will not be a real health problem."

The protein from the Moringa Oleifera seeds can also help companies extract valuable minerals for the mining process, according to Rennie. He said that his team has recently been conducting work on using the protein as a way of separating minerals during the flotation process. "One of the big issues in mining has been that you typically end up with low yields of ores (1%) of what you are interested in," he says.

Rennie adds that the protein can bind to certain materials and not to others, in order to effectively separate valuable minerals. Nevertheless, he said that this process will not "necessarily be replacing all of the processing materials" that companies use, because "typically, they might be using a range of materials to process your water depending on what you want to achieve".

All in all, there are numerous benefits to using minerals in water and wastewater treatment. Lime, magnesia and caustic soda products all have their specific advantages. With the world’s population set to increase to 9.6bn by 2050, it is inevitable that more pressure will be put on the world’s waterways. Put simply, there will be more waste and an increasing need to provide clean drinking water. Minerals can help give the water of life.