Hazardous chemicals: what's the way forward?
Tightening standards on chemicals in surface waters mean a re-examination of the removal of hazardous chemicals during wastewater treatment, say Bruce Petrie and Professor Elise Cartmell of Cranfield Water Science Institute
The European Union has established environmental quality standards (EQS) for hazardous chemicals present in surface waters as detailed in the Priority Substances Daughter Directive (Directive 2008/105/EC). Due to these increasingly stringent regulations and wider ranging legislation relating to water and the environment the drive now is to determine appropriate control measures which can include the removal of hazardous chemicals during wastewater treatment.
To ensure environmental protection there is a need to examine a variety of chemicals including metals, priority hazardous substances, polycyclic aromatic hydrocarbons, pharmaceuticals, steroid estrogens and emerging substances such as triclosan. Such a wide variety of organic and inorganic chemicals offer extremities in physio-chemical properties. This causes their fate and behaviour to vary significantly during wastewater processing, making optimisation of their removal simultaneously difficult.
Rivers are recipients of wastewater discharges which can contain a ‘cocktail’ of these hazardous chemicals typically in the ng l-1 to µg l-1 range. Therefore, limiting the entry of these substances into the wastewater stream is highly desirable. However, for some hazardous chemicals this is not possible (for example, steroid estrogens and pharmaceuticals) due to their natural production or essential medical usage. Restrictions on the use of some chemicals (such as nonylphenol surfactants and the use of lead in petrol) have seen a substantial reduction in concentrations of some hazardous chemicals in surface waters over recent years. Despite this, these chemicals can still be detected in concentrations above their EQS.
Even at low ng l-1 concentrations, some chemicals can breach their respective EQS and be of environmental concern. To demonstrate, an active ingredient in contraceptive medication is the synthetic estrogen 17α-ethinylestradiol (EE2); this chemical has a proposed EQS of 0.035 ng l-1 due to its high estrogenic potency. Reaching such low concentrations is difficult due to the recalcitrant nature of EE2. Elucidating whether or not EE2 actually exceeds its EQS is troublesome and may be controversial when these compliance measures are implemented as such concentrations test the capabilities of current analytical methodologies.
Without adequate removal during wastewater treatment, or suitable dilution by the river, an EQS failure can be caused within the riverine system for any given hazardous chemical. This is particularly pertinent for rivers with low dilution ratios and those which have numerous effluent discharge sites along their catchment. Thus focus has been placed on their removal during wastewater treatment and possible strategies for improving their removal. Such is the concern that the UK Water Industry Research (UKWIR) is managing the Chemical Investigations Programme (CIP) to support the determination of appropriate pollution control measures.
A possible approach to enhance removals is the addition of a ‘train’ of tertiary processes (more typical of drinking water applications) to an existing conventional wastewater treatment system. Options such as microfiltration reverse osmosis (MF-RO), ozone and granular activated carbon (GAC) are all known to be effective in removing hazardous chemicals.
However such processes are controversial as they can come at a significant cost, both financially and to the environment as a whole through increased carbon outputs. Capital outputs for the construction of such processes are high. Further costs include their maintenance, higher energy consumptions and the use of consumables (such as chemicals for dosing) which can make the cost of wastewater treatment significantly higher. The costs of which would be passed onto the customer which, in the current economic climate, are unfavourable. Such an approach also produces an inflated carbon footprint of the treatment process. Therefore affording greater protection to aquatic biota will in fact be detrimental to the whole environment.
A more ‘sustainable’ approach is therefore to focus efforts on the optimisation of existing processes and the possibility of additional complementary low energy tertiary processes.
Extensively used secondary processes such as activated sludge are designed primarily for organics and nutrient removal. However, these can fortuitously remove significant concentrations of hazardous chemicals.
Optimisation of such processes to enhance their removal collectively is highly desirable as this could reduce the need for more controversial tertiary processes.
Understanding the impact of operational conditions to the simultaneous removal of all hazardous chemicals is currently lacking. Furthermore, the impact of this manipulation of operational/design parameters to optimise process performance on the microbial communities responsible for removing hazardous chemicals is not clear. Unless such an understanding is reached, the systematic and predictive manipulation of the conditions required to optimise removal of hazardous chemicals will be problematic.
An extended solids retention time (SRT) is well known for augmenting removal of those chemicals which are amenable to biodegradation such as the steroid estrogens. Despite this, the impact of shifting SRT to inorganic chemical removals (such as metals) is currently unknown. SRT offers the most straight forward way of modifying the operation of existing works.
Therefore identifying a SRT which provides maximum removal of all the hazardous chemicals would be a significant step towards producing a sustainable treatment strategy capable of producing effluents qualities which comply with EQS limits.
It must be noted that operation at a greater SRT (and concomitantly at a greater mixed liquor solids concentration) will incur greater aeration costs to maintain complete mixing. However, these costs should be significantly lower than the capital and consumable outputs of advanced tertiary processing.
The impact of hydraulic retention time (HRT) to hazardous chemical removal remains unclear. Operation at an extended HRT (and SRT) to maintain a low food: microorganism ratio (F: M) will improve contact time for sorption and biodegradation to occur.
Further possible options for existing process optimisation is in the design of primary treatments (for example, sedimentation). Over-sizing of the primary sedimentation tank to increase settling time and remove additional solids will improve removal of those hydrophobic chemicals which are associated with particulate matter in the crude stream. This would be particularly advantageous for metals whose concentration in the crude stream is mainly associated with particulates.
The final available option is the addition of a low energy tertiary process such as trickling filters. Further hazardous chemical removals here could be driven by the reduction in carbon competition; a theory which needs to be tested.
Secondary effluent consists of a higher hazardous chemical: bulk organic ratio than that observed in the crude stream. Preferential biodegradation towards bulk organics (as observed in secondary treatment) will not be able to proceed due to their absence in secondary effluents.
As a result the bacteria in the trickling filter become ‘forced’ into biodegrading those less favourable sources of carbon – organic hazardous chemicals.
Competition for sorption sites on the bacterial biofilm surface will also be reduced, enhancing removal of non biodegradable hazardous chemicals.
As discussed, there are a number of possibilities for low energy low cost existing process optimisation. These need to be explored in depth and optimised at pilot and lab scale before transferral to full scale works. This could offer a relatively ‘sustainable’ approach to reaching EQS compliance targets to ensure environmental protection without the need of high energy high cost tertiary processes.
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