Oct-2012

Innovations in wastewater treatment

Industrial gases provide a wide range of options for treating wastewater from petrochemical processes

DARREN GURNEY
Linde Gases Division

Article Summary

The demand for efficient and cost-effective wastewater treatment technology in the refining and petrochemical sector is being driven by not only ever-tightening environmental legislation, but also by the sector’s own desire to follow a meaningful sustainability agenda and to take its responsibilities around product stewardship seriously. However, refining and petrochemical companies are continually confronted with the challenge of striking a balance between making their activities profitable while ensuring the industrial processes involved in the production and application of a chemical product, across its lifecycle, have minimal impact on the environment.

Treatment of wastewater from petrochemical plants can be a challenging and costly matter, particularly when needing to comply with the requirements of operational permits and national environmental legislation governing the discharge of treated wastewater into community treatment plants or natural water bodies such as rivers, lakes and oceans. The segregation, collection and treatment of wastewater play a vital part in the protection of public health, water resources and wildlife. Refining and petrochemical facilities, as part of their permit to operate, must demonstrate that they are successfully able to treat all their pollution streams to the appropriate regulatory standards.

One of the most widely used strategies to meet the ever-rising demand for water and increasingly strict regulations governing water protection is through improved water management and strengthened investment in the technologies that preserve and recycle process wastewater.

The refining industry converts crude oil and associated petroleum gas (APG) into hundreds of refined products, including petroleum, diesel fuel, kerosene, aviation fuel, fuel oils, lubricating oils and primary feedstock for the petrochemical industry, and in doing so it employs a wide variety of physical and chemical treatment processes in which large volumes of water are utilised, especially for cooling systems, distillation, filter backwashing and deionisation techniques. Vessel cleaning, equipment flushing and surface water run-off also generate additional volumes of wastewater that need to be treated. In turn, the petrochemical industry produces a multitude of essential products to modern-day living including intermediates for the pharmaceutical industry, aromatic organics, solvents, alcohols, ketones, polymers and aldehydes, all of which are synthesised through various process operations, which can produce large quantities of wastewater that must be treated.

Given the complex and diverse nature of refinery wastewater pollutants, a combination of physical, chemical and biological process trains and treatment methods are usually required before it is finally discharged into the aquatic environment.

Wastewater treatment can be improved significantly by harnessing industrial gases such as pure oxygen, for example, to enhance the biological assimilation and oxidation processes of wastewater treatment plants or prevent undesired odours in refinery mains or storage lagoons and tanks. Carbon dioxide is a versatile and safe substitute for corrosive mineral acids to effectively neutralise alkali wastewater.

The technology used for refinery wastewater systems is site specific and depends on the nature of influent (incoming wastewater) conditions and the level of treatment required by local regulatory authorities. However, a typical refinery wastewater treatment plant usually consists of physico-chemical pre- and primary treatment, followed by secondary biological treatment and tertiary treatment, if necessary.

In a refinery wastewater treatment system, two steps of oil removal are typically required to achieve the necessary removal of free oil from the collected wastewater prior to feeding it to a biological system. This oil removal is achieved by using an American Petroleum Institute (API) or equivalent oil water separator followed by a dissolved air flotation (DAF) or induced air flotation (IAF) unit.

The wastewater is then routed to the primary treatment clarifier and to the aeration tank and secondary clarifier, which constitutes the biological system. The effluent from the clarifier is then sent to tertiary treatment, if required, prior to discharge. The activated sludge process is the most widely used wastewater treatment technology for the removal of soluble organic contaminants in the oil refining and petrochemical industry. Often the pH of the raw wastewater requires reducing before it can be accepted by the bio-treatment stage, as the high pH could potentially kill off the bacteria doing the treatment.

CO2: versatile acid alternative for pH control
In the UK, the industrial gases technology company BOC Ltd, part of The Linde Group, has seen wastewater treatment successfully implemented at a major plant operated by one of the world’s leading petrochemical manufacturers. 

The 1700-acre site is highly integrated, exploiting synergies between the petrochemicals plant and adjacent refinery. The petrochemicals facility manufactures over 2 million tonnes of chemicals products per annum and the refinery has an annual capacity of 10 million tonnes.

At the petrochemicals plant, an environmentally friendly CO2-based technology, Solvocarb, is being used to control alkali aqueous wastewater pH prior to discharge. The system uses gaseous CO2 to neutralise alkaline waters through the production of carbonic acid (see Figure 1).
The refinery, in compliance with legislation at the time, had been discharging wastewater from the plant into the local river estuary after adjusting its pH using mineral acids, such as sulphuric and hydrochloric. Variability in discharge pH and the corrosive nature of strong mineral acids led to concerns over potential harm the discharge may cause to aquatic wildlife resident in estuaries. 

In addition to the plant needing to find a more environmentally friendly wastewater pH control solution, it needed to find one that would give it more robust control over the whole process. In order to achieve the target pH range through the use of mineral acids, the company observed periods of pH oscillation from too much acidity dosing, requiring adjustment with additional alkalinity. This inevitably leads to extra cost and operating complexity arising from operating two pH adjustment processes. The company ultimately opted for a single process route involving CO2, which preserves the natural alkalinity of the wastewater and the process pH control is more stable over the desired pH control range. BOC was appointed to design the pH control system for the newly designed wastewater treatment plant.

Owing to strict environmental permits, wastewater may only be discharged into the outlet channels if it is within a narrow pH range —usually between 9 and 6. The Solvocarb method employs gaseous CO2 to neutralise alkaline waters. When dissolved in water, CO2 forms carbonic acid, which reacts with the alkalinity to form a salt. The neutralisation reaction controls the pH value to the appropriate discharge level.

Awarded the contract to design two Solvocarb systems to neutralise all plant wastewater, anywhere between 10 000 and 20 000 cu m/d, BOC engineers designed each system to simultaneously mix and dissolve CO2 into each 10 000 cu m tank, controlling the pH to the appropriate set point. Consent permits dictated the site could only discharge pH-corrected wastewater into the estuary when the tide was in, allowing the treated wastewater to dilute effectively into the larger body of available water in the estuary. However, this meant that the whole process had to be completed within a restricted time period of a six-hour window between the two tides.

It was critical for the wastewater to be neutralised in the two tanks within the time available, which called for challenging process hydrodynamics. Large and variable volumes of wastewater needed to be brought within the correct pH range within a fixed timeframe — the wrong pH value could result in the refinery being unable to discharge the wastewater, causing potential bottlenecks and resulting in backups further up the process chain. A significant amount of testing was conducted before the team was satisfied that the proposed system would operate to the required parameters.

The new wastewater treatment plant was commissioned in February 2000 and was completed on time and on budget. BOC has continued to supply the plant for nearly 12 years. The continued successful operation of the plant will help safeguard the environmental status of aquatic areas in which the wastewater is discharged.

Today, the main driver for treating effluent high in alkalinity prior to discharging to the outfall is the strict regulation to protect the sensitive, biodiverse ecosystem within the estuary. Using CO2 to neutralise an alkali effluent avoids large swings in the discharge pH, a vital component in creating a sustainable and suitable environment for marine life.

Compared with mineral acids commonly used in previous years, CO2 offers many advantages, amounting to the best economical and ecological alternative. CO2 is not categorised as a substance that is harmful to water and does not lead to the addition of unwanted anions in the water environment, such as chlorides and sulphates. There is also no over-acidification of the wastewater, due to the self-buffering nature of CO2 in water, which produces a flat neutralisation curve and no corrosion of system and equipment components. CO2 is also far safer than the acids previously used. Simple to handle, it is delivered as a liquid cryogen that is stored in tanks onsite and dosed automatically into the process.