The alarming degradation of receiving waters in the United Kingdom and Ireland has the environmental agency, together with the municipal water company’s, looking for ways to limit priority substances and other pollutants. While non-point source controls are slowly emerging, point source wastewater dischargers are already feeling the effect of more stringent effluent discharge consents for nutrients (phosphorous and nitrates).
There are few technologies available to help treat for priority substances. The most promising technologies include Blue CAT™ reactive sand filtration, an advanced oxidation process (AOP). The Blue CAT™ AOP is essentially a super-charged Blue PRO™ reactive filtration process offering a unique approach to priority substance mitigation by combining chemical and physical treatment in one unit.
The concept is to increase the contact and oxidation potential of chemical treatment by coating a high surface area media with a temporary reactive chemical coating in a moving bed sand filter. The Blue CAT™ process oxidises and denatures molecular structure, and it provides a physical barrier to downstream pollution. A brief overview of the process is beneficial for process integrators.
Technology description
The Blue CAT™ system has multiple aspects effecting the end-goals of treatment. Ferrous and/or ferric salts are added to the Blue CAT™ system with ozone. There are many reaction pathways that happen in parallel, but the critical pathways for treatment of priority substances include those in which hydroxyl radicals are formed elevating the oxidation/reduction potential (ORP) to >900 mV. The following is an example pathway for treating organic compounds of concern:
Precursor complex formation:
Equation 1: ºFe
IIIOH + O
3 ® ºFe
III–OH–O
3
Equation 2: ºFe
IIIOH + Organic(M) ® ºFe
III–OH–M(ºM)
Electron transfer:
Equation 3: ºFe
III–OH–O
3 ® ºFe
II–O
2 + HO
2·
Equation 4: ºFe
II–O
2 + H
2O ® ºFe
III–OH + HO
2·
Equation 5: ºFe
II–O
2 + ·OH ® ºFe
III–OH + O
3
Generation of hydroxyl radical:
Equation 6: O
3 + HO
2· ® ·OH + 2O
2
Oxidation of organic compound:
Equation 7: OH· + M ® ºM
oxidized ® CO
2 + H
2O
[caption id="attachment_26858" align="alignright" width="300"]
Figure 1: Reactive filtration media receives a temporary hydrous iron salt coating[/caption]
The elevated oxidation potential is further enhanced by increased contact probability within the system. Similar to Blue PRO™ reactive filters for ultra-low phosphorous removal to 0.1 mg/L of Total Phosphorous, the reactive media coating in a Blue CAT™ AOP process plays a vital role.
Media within the surface has an incredible surface area ensuring contact with the wastewater being treated. All of the media surface area becomes coated with the iron salt complexes, creating a catalytic surface area for the AOP process.
Figure 1 includes images from a scanning electron microscope showing the sand grains coated with a hydrous iron salt that. This is a temporary coating formed
in situ. The coating is subsequently stripped and replaced
in situ in cycles ranging from three to six hours by nature of the filtration platform. This aspect of the process is also important in preventing surface overload conditions.
[caption id="attachment_26859" align="alignright" width="300"]
Figure 2: The surface area of the media allows for one of the most advanced AOPs for priority substances[/caption]
The final aspect of the process is the physical barrier to non-dissolved constituents and solids. The Blue CAT™ AOP is provided in filter platform, and the benefits of filtration are realised as well. The filters provide reuse quality treatment.
Like the Blue PRO™ process, this AOP maintains mechanisms of adsorption, and does not clog the filter media with flocculated particulates. The media bed is described as open to flow and has a low relative headloss compared to diffusion processes using coagulation combined with filtration processes.
History and use
Blue PRO™ systems and the related Blue CAT™ OAPs have seen industrial use over the last decade with ongoing process development. Removal of nutrients, heavy metals, and other priority substances has been modeled with observations on disinfection and color reduction.
Heavy metals and nutrients, including phosphorus, are removed in the Blue CAT™ AOP. This process has been shown to meet even the most stringent phosphorous limits with permitted municipalities operating < 0.02 mg/L P (Citronelle, Alabama, USA). An installation at Marlborough, Massachusetts, USA, is a great example of a system that consistently meets stringent TP limits using reactive filters.
This facility has a design flow of 45,000 m
3/day. Future Total Maximum Daily Load (TMDL) for phosphorous was established requiring a future capability of 0.07 mg/L P for summer operations. CDM Smith in conjunction with Blue Water Technologies, Inc. completed the installation in late 2011. The current winter TP limit is 1 mg/L P, and the current summertime TMDL requires 0.1 mg/L phosphorus as shown in Figure 3.
[caption id="attachment_26861" align="alignright" width="300"]
Figure 3: Marlborough Massachusetts monthly compliance data[/caption]
Data in Figure 3 shows the monthly averages complying with requirements during the first three years of full-scale operation. Summer effluent TP values between 0.03 and 0.06 mg/L for this single-stage treatment system. This installation is representative of other sites operating in the western hemisphere.
The target in the UK has been to deliver an economical process to maintain phosphorus < 0.1 mg/L of Total Phosphorous. Blue PRO™ plant modeling for phosphorus removal has been conducted in Ireland and the United Kingdom treatment sites include Horwich Sewage Treatment Plant (STP), Windermere STP, Grasmere STP for United Utilities and Llanberis STP for Welsh Water.
The next phase of testing at Horwich STP is a larger scale Blue CAT™ AOP being commissioned in early 2016 for modeling of priority substances. The commissioning is being completed by Evergreen Water Solutions and the modelling is be facilitated by United Utilities over a 12-month period.
Installations of the Blue CAT™ process will record the following benefits, some of which will be monitored during operations at Horwich STP:
- Heavy metals control: mercury to < 1 ng/L, copper to < 5 µg/L, lead to < 1 µg/L
- Ozone concentration
- Colour removal
- Disinfection has been observed. Figure 3 shows total coliform results from a 1 L/s pilot operated during the development of this process. The figure illustrates 3.5 log disinfection of fecal coliforms to <2 cfu/100 mL.
[caption id="attachment_26862" align="alignright" width="300"]
Figure 4: Disinfection studies illustrating discharge with and without the Blue CAT™ AOP from HARSB activated sludge plant, Hayden, Idaho, USA[/caption]
Reactive filtration methods are so appealing because they meet the immediate needs for many facilities while providing a platform for future flexibility. The last thing that any municipality wants to do is install a system that is a temporary solution, only just sufficient to meet the current pressing needs.
Forward thinking consultants have caught on and consider reactive filtration to mitigate current treatment goals while providing future flexibility and turndown for future permit modifications. The fact that reactive filters also produce reuse quality water at a low cost is added incentive to consider this method of trace contaminant removal since the system represents a physical barrier to contaminants in discharge.
Overall, this is a safer, more flexible, and more consistent approach than any dynamic sedimentation model. The Blue CAT™ AOP has exciting potential as a solution for priority substances