Going green: chemical wastewater treatment

Well-treated wastewater has enormous potential as a source of water for crops, households and industry. Rowan Watt-Pringle investigates whether new chemical treatments are providing green solutions for wastewater recycling.

As rising populations and the spread of industry increasingly place stresses on the world's water resources, the wastewater produced by many sectors - from municipalities to mining and power generation processes - represents a vital potential source of recycled fresh water.

There are various processes for treating wastewater, which vary according to the types of contaminants found in the water and include physical, biological and chemical processes, as well as different combinations of the three.

"Adsorption is an example of a process that straddles both physical and chemical processes."

However, wastewater treatment has traditionally been a high consumer of energy, producing significant carbon dioxide (CO2) emissions, meaning the development of new environmentally-friendly processes is a large factor in the mandate of companies providing wastewater treatment chemicals and/or technologies.

According to Ruediger Knauf, vice-president of Siemens Water Technologies' (SWTs') Global R&D: "What is needed to address growing challenges like water scarcity and climate change is a holistic approach for water and wastewater treatment."

For this reason, SWT is conducting R&D aimed at increasing process and energy efficiency. "The answer lies in technology," says Knauf. "It has to help secure water supply for the public and industry in the desired amount and quality, and is expected to do so at the lowest cost and highest level of environmental sustainability."

Chemical wastewater treatment variations

There are various chemical wastewater treatments available to industrial, mining and other sectors, including chlorination, neutralisation, ion exchange, coagulation and adsorption.

Many of these treatments use the oxidation process to purify wastewater, which is explained by the American Institute of Chemical Engineers (AIChE) as "the addition of oxidising agents - chemical ions that accept electrons," through a variety of advanced oxidation techniques.

According to the AIChE, chlorine-based oxidation is highly effective and has been the most widely adopted oxidation technique, although "the transportation, storage and use of chlorine (which is highly toxic) present significant potential health and safety risks during water treatment operations."

For this reason, there has been a move toward developing safer and often more effective oxidation techniques. The AIChE highlights ultraviolet light, hydrogen peroxide and ozone as powerful oxidising agents that destroy unwanted contaminants and disinfect treated water without the inherent risks of using chlorine.

"However, wastewater treatment has traditionally been a high consumer of energy."

Neutralisation is a process fairly common to industrial wastewater treatment, whereby acid or base (such as lime) is added to the water to return the pH value to neutral, while iron and other polyvalent metals (metals that can form multiple covalent bonds with other elements) are frequently used as coagulants in the coagulation process.

Also known as flocculation, this process is used to separate suspended solids from contaminated water. The Minnesota Rural Water Association (MRWA) states the choice of coagulant chemical depends on the type of suspended solid, as well as other factors including raw water conditions and facility design, stating: "Common coagulant chemicals used are alum, ferric sulfate, ferric chloride, ferrous sulfate and sodium aluminate."

Adsorption, on the other hand, is an example of a process that straddles both physical and chemical processes. Carbon has long been used to adsorb waterborne contaminants, the atoms, ions or molecules of which adhere to the surface of the carbon.

As the AIChE explains, when charcoal is heated it forms a highly adsorbent form of carbon - activated carbon - which has an extremely intricate internal-pore structure, offering a phenomenal internal surface area. According to the AIChE "just 5g of activated carbon has the surface area of a football field," greatly enhancing its adsorbent characteristics.

Advances in chemical wastewater treatment

As the wastewater treatment industry persists in striving for enhanced environmental and cost benefits, innovative R&D relating to chemical wastewater treatment continues apace.

Siemens Water Technologies - as a leading carbon adsorption technology provider through its Westates line of activated carbons and equipment - announced in July it would provide bulk carbon media replacement and carbon reactivation services at a wastewater treatment plant in Thailand.

Siemens designed, engineered, manufactured, supplied and constructed the wastewater plant (as well as another water plant at the Thailand Growth Project's Asia Industrial Estate in the city of Rayong), which began operations in February, treating up to 5,500m³/day of process wastewater from the estate's chemical manufacturing facilities to meet discharge limits.

The treatment plant incorporates biological treatment using jet aeration, clarifiers, sand filters, four activated carbon filters and a reactivated carbon storage vessel, while in a July press release, the company asserts that: "Because the carbon is reactivated and then reused, with the organic waste destroyed, it's an environmentally sound solution."

In June, SWT also introduced the AquaCarb CX Series of coconut shell carbon for surface water treatment. SWT says: "The product can be considered as a new alternative to coal-based activated carbon for surface water treatment, where taste and odour removal, disinfection byproduct (DBP) or DBP precursor removal, and total organic carbon (TOC) removal are required. The product also serves as a premium performance choice for groundwater applications."

The company sees this as an important step forward, as carbons in the AquaCarb series "have the high microporous structure of coconut shell-based carbon combined with the faster kinetics of bituminous coal-based carbons, resulting in excellent volatile organic carbon (VOC) removal capacity, while also working well in applications where bituminous coal-based carbons have been the preferred choice."

Klean Earth Environmental Company (KEECO), meanwhile, created a process known as silica micro encapsulation (SME) just more than a decade ago for the removal of heavy metals from wastewater. SME remains one of the most effective ways of dealing with this type of contaminant because, according to company researchers: "Unlike conventional neutralisation or precipitation methods, SME encapsulates the contaminants in a permanent silica matrix resistant to degradation under even extreme environmental conditions."

This means "encapsulated metals are effectively immobilised, minimising the potential for environmental contamination and impacts on human or ecosystem health."

Incorporating physical and chemical elements, the process also reduces dissolved solids like sulfates, while high-energy oxidation degrades or destroys harmful hydrocarbons and other organic chemicals.

The environmental benefits of such a process can be summed up by the fact that its most common application has been to treat acid rock drainage (ARD) from both operational and abandoned mines and quarries.

Large amounts of wastewater are produced by the processes undertaken at coking plants (which are vital to the iron and steel industry). With this in mind, researchers Hongqiang Ren, Yunjun Yang, Lili Ding and Xiaolei Shi at China's Nanjing University have this month applied for a patent relating to a new method for dealing with the high concentration of ammonia nitrogen in coking wastewater.

The new physiochemical method integrates electrochemical oxidation and flocculation processes to pre-treat the wastewater by adding magnesium and phosphate, which combine with the ammonia nitrogen to form magnesium ammonium phosphate (MAP), thereby removing ammonia nitrogen from the wastewater. This allows a secondary, biological process to be instigated, which requires lower levels of ammonia nitrogen to work effectively.

"There are various processes for treating wastewater, which vary according to the types of contaminants."

As Hongqiang et al point out: "This method can significantly reduce major pollutants in coking wastewater and improve its biodegradability." By adding fly ash into MAP during its pyrolysis process, the method on one hand lowers the process temperature, expedites pyrolysis time and improves ammonia gas release, while on the other hand simultaneously cutting down on the cost of sodium hydroxide (required for conventional MAP decomposition techniques) and achieving their stated goal of beneficially utilising alkaline fly ash.

In conclusion, Vancouver Island's wastewater resource for Canada's Capital Regional District (CRD) core municipalities stresses that efficient wastewater treatment innovations should be flexible, allowing new innovations and technologies to be employed as they become available.

They should also be able to integrate with other local services including solid waste, energy and public services to maximise the benefits of wastewater treatment, as well as lower costs through waste recovery and reuse while minimising negative environmental impacts, while a final consideration to take into account is the need to integrate processes across municipal and industrial sectors to maximise environmental and cost benefits.