FeatureFiltration+Separation April 2008

Membrane chemical reactors:

Treating difficult fluid wastes

r. James Ellis explains how a new development by Water Innovate, called a membrane chemical reactor, can treat a variety of recalcitrant fluid wastes in the industrial sector and in water reuse and municipal applications.

Introduction

In this article we explain how membrane chemical reactor (MC-RTM) technology can significantly reduces the hard chemical oxygen demand (COD) content in the recalcitrant fluid waste, which normally remains untreated by conventional biological treatment systems. The process utilises the chemical, titanium dioxide (TiO2), as a photocatalyst within a closed-loop UV system, the chemical being initially mixed with the waste prior to UV treatment, and then selectively separated using advanced membrane technology. The chemical is not consumed but constantly recirculated and reused in the process.

Recalcitrant COD currently imposes significant problems in certain industrial sectors; innovative MC-R technology can help treat such fluid waste on-site and reduce the significant disposal costs otherwise incurred by improving the quality of the waste discharged or by reusing treated effluent within the facility.

Treatment and reuse

Regulatory controls over industrial waste disposal are becoming increasingly strict in many countries, especially within the European Union (EU). The current regulatory regime emphasises not only tight waste

quality discharge controls, which can impose substantial waste treatment costs on operators, but also encourages waste minimisation and energy efficiency improvements.

High-quality on-site waste treatment where possible is considered BAT (Best Available Technology) under UK Integrated Pollution Prevention and Control (IPPC) legislation, since this opens the possibility of waste-stream reuse within the process whilst removing the necessity to transport hazardous wastes safely.

There are numerous sectors that generate fluid wastes that are significantly more difficult to treat, even with these established technologies.For many industrial sectors technologies such as membrane bioreactors (MBRs) and other advanced wastewater treatment systems have enabled the adoption of on-site treatment or permitted the upgrade of existing treatment systems to meet new requirements in a space-efficient and cost-effective way. However, there are numerous sectors that generate fluid wastes that are significantly more difficult to treat, even with these established advanced technologies.

Industrial sources of recalcitrant waste where high levels of hard COD are present (typically ~100 000 mg/l) include, for example, landfill (see Figure 1), pharmaceutical, metal working, stone cutting, grinding and aircraft washing.

Such recalcitrant waste problems have traditionally required the collection, storage and transport of liquid waste via tanker to specialist waste treatment contractors, all of which impose significant costs to industry. These costs are set to increase as high fuel prices are passed on by transporters to their customers. This current situation is becoming

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Figure 1: MC-R technology can be applied in a number of industries, including landfill leachate treatment, for the reduction of hard COD.

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unsustainable both environmentally and financially. Environmentally, on-site treatment is the preferred approach. Financially, the costs to operators are significant. For example, the UK generates 400,000 tonnes of metal working fluids (MWF) annually; tanker costs alone are around GBP 100/tonne and set to rise. These costs impose a competitive constraint on business in this particular industrial sector.

Clearly, there is a need to develop technologies capable of treating recalcitrant fluid wastes on-site. Initial capital outlay will quickly be recouped through transport and disposal cost savings and the treated waste can be discharged much more cheaply to sewer under the appropriate licenses for subsequent treatment and discharge by the local wastewater treatment company.

The MC-R solution

MC-R technology works to treat COD that is resistant to conventional treatment technologies using a combination of ultraviolet (UV) light and the photocatalyst, titanium dioxide (TiO2). UV treatment is well known for its capability to completely oxidise organic matter, including the aromatic compounds normally resistant to conventional biological treatment, because of their inherent chemical stability or toxicity

to biological systems. Without treatment, this resilient organic waste component can leave recalcitrant wastes with unacceptably high COD after conventional treatment. UV treatment affects the rapid breakdown of ring-structure (cyclic and heterocyclic) and other chemically resistant organic compounds, ultimately to carbon dioxide via various chemical intermediates.

Water Innovate’s MC-R technology comprises three unit processes within a single engineered unit as shown in the process schematic (Figure 2). Firstly, TiO2

is contacted with the waste. This is then pumped through the UV process where the TiO2 acts as a photocatalyst. UV light excites electrons on the surface of the TiO2molecules where they react with adsorbed oxygen to form oxygen radicals. Hydrogen ions also migrate to the surface of the TiO2molecules where they react with water to produce hydroxyl radicals. The result is the production of a highly oxidising environment from a durable photocatalyst that does not require frequent replacement. The typical operating costs of MC-R technology compare

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Figure 2: Typical schematic of Water Innovate’s MC-R system. Key: 1 = Contactor (TiO2 photocatalyst mixed with waste), 2 = UV reactor, 3. = membrane separation of TiO2 and decontaminated effluent.

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FeatureFiltration+Separation April 2008

favourably with other types of oxidation technology as detailed in Table 1.

Following the UV oxidation step, specialised advanced membrane technology is used to selectively separate the TiO2 from the treated waste. The TiO2 photocatalyst is then transferred back to the start of the process for continuous reuse. The result is zero consumption of chemicals in the MC-R process. The only input required is electricity to power the UV units, pumps and controls. Maintenance requirements are low, with only periodic cleaning of the membrane component required. The MC-R process is supplied as an integrated unit comprising all instrumentation and control equipment.

UV treatment is well known for its capability to completely oxidise organic matter, including the aromatic compounds normally resistant to conventional biological treatment.Both UV and TiO2 treatment potentials have been well researched over the last decade, especially at the Centre for Water Science, based at Cranfield University in the UK. MC-R technology was initially developed at Cranfield and Water Innovate (with a patent on the technology) has now developed the treatment approach to permit its implementation to

the target industries. Treatment performance is excellent, effectively reducing recalcitrant effluents with COD ~100 000 mg/l to <1000 mg/l, a level suitable for discharge to sewer. MC-R technology has successfully treated a range of wastes across the various industrial sectors identified above at different scales to meet individual site requirements.

The development of the MC-R waste treatment solution was assisted by an award of GBP 1M from The Department of Trade and Industry (UK government) and GBP 2M from the EU to the consortium, including Water Innovate, for the development of membrane chemical reactor technology.

Conclusions

Hard COD in wastewater can be difficult to treat even using newer biological treatment systems. Some industrial sectors, for example landfill, pharmaceutical and metal working, still resort to the collection, storage and removal

of their fluid wastes for off-site specialist treatment. This is expensive, and does not fit easily with current regulatory preferences for on-site treatment (where possible) which can enable waste reuse and reduce the environmental impact and risks of transporting large quantities of hazardous waste.

To this end, Water Innovate has developed its patented MC-R technology to effectively treat hard COD without the consumption of chemicals. On-site treatment of recalcitrant waste is now feasible, and can improve both the financial and environmental performance of the target industries. The treatment process can be scaled to suit each individual site requirement, and pilot testing can be undertaken to determine the optimum MC-R configuration prior to installation. •Contact:Dr. James Ellis, Commercial Manager Water Innovate Ltd. Tel: +44 (0) 1234 758054. www.waterinnovate.co.uk

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Table 1: Comparison of estimated operating costs (USD per litre) between carbon adsorption, UV / Ozone and UV photocatalysis.

System Size (MGD) 0.029 0.058 0.115 0.23 0.46 0.92 2.44

Carbon Adsorption 4.9 3.4 2.7 2.0 1.7 1.4 1.2

UV / Ozone 8.0 5.3 4.0 3.1 2.7 2.4 2.0

UV Photocatalysis 6.2 4.0 2.7 2.0 1.6 1.5 1.3

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