Arvia Technology has introduced a new process for the removal and destruction of organic contaminants in water. This article is a summary of a presentation given at IWEX by Nigel Brown, the company’s technical director.
These pesticides are some of the most refractory compounds and have long been suspected of causing diseases such as leukaemia. The European Union 1998 Drinking Water Directive specifies a limit of 0.1 µg/l for individual pesticides with a maximum pesticide total of 0.5 µg/l. One pesticide that has increasingly been detected at levels above 0.1 µg/l in UK potable water sources is metaldehyde.
Metaldehyde (CH3CHO)4 is a molluscicide sold under a variety of trade names for control of slugs, snails and other gastropods. Whilst only slightly to moderately toxic to humans, it still falls under the definition of pesticides so must be controlled to below 0.1 µg/l.
The problem facing water companies is that metaldehyde passes through existing treatment process relatively untreated, including activated carbon. Arvia Technology Ltd, a spin out from the University of Manchester's School of Chemical Engineering, is commercialising what it describes as a revolutionary new approach for the removal and destruction of organic contaminants. Arvia's process uses adsorption coupled with the electrochemical regeneration of that adsorbent. This process is based on the properties of a novel carbon based adsorbent, Nyex. The important properties are that the Nyex is:
· Non-porous - so that adsorption is rapid as there are no internal pores (15-30 minutes), but this means that it is a low capacity adsorbent
· Dense - allowing the particles to settle quickly (5-10 minutes)
· Highly electrically conducting - resulting in the rapid and low-powered regeneration of the adsorbent through the oxidation of the organics to carbon dioxide and water (5-45 minutes).
Thus the adsorbent is typically regenerated 20-25 times a day, so although it is a low capacity adsorbent, the system will have a similar capacity to a GAC (Granular Activated Carbon) system after around 20 days operation.
These Nyex properties have allowed Arvia to develop a process where the three unit operations of adsorption, separation and electrochemical regeneration can be achieved in a single vessel.
Arvia has developed both batch and continuous processes, using fluidisation for adsorption, gravity sedimentation for separation and simple electrochemical cells for low power regeneration. Hence there are no moving parts within the treatment tank.
The problems with the presence of pesticides in water have resulted in UKWIR commissioning the WRc to undertake a project into the treatment of Pesticides (to be published by UKWIR as "DW14: Treatment for New and Emerging Pesticides" by Glenn Dillon et al., 2011).
As part of this project Arvia was supplied with a number of waters spiked with pesticide. Partially treated (pre-ozonated) water from a water utility was spiked with single pesticides and deionised water was spiked with a range of pesticides. Group 1 was spiked with metaldehyde, clopyralid, MCPA and mecoprop and Group 2 with metaldehyde, quinmerac, carbetamide and propyzamide.
These waters were treated in Arvia's laboratory scale test system treating a batch of 1l per cycle. Several cycles were run to generate sufficient sample. The regeneration energy used to regenerate the adsorbent was calculated as 0.1 and 0.27 kWh/m3 for the low and medium intensity treatments respectively. The results obtained are shown in the table below.
These results are in line with results obtained from other trials which show preferential removal of pesticides compared with other organics. The figure above shows the treatment of a highly polluted wastewater containing an unidentified pesticide (measured by HPLC peak area). This figure shows that there is a rapid removal of the pesticide, whilst there is a smaller reduction in the overall organics removal as shown by COD. The figure highlights the linear nature of the treatment process due to the concentration of the organics onto the adsorbent surface and that continuing treatment can achieve full removal of the organics.
The Arvia process has been proven effective and is making inroads into a broad range of markets, including groundwater remediation, swimming pool water treatment, and the treatment of radioactive oil wastes in the nuclear industry. Arvia also continues to receive recognition for its technology, recently winning the highly prestigious Rushlight Clean Environment Award.