Supercritical UV for Disinfection of Low UVT, High Turbidity Wastewater Effluent

Introduction
Many water jurisdictions worldwide have chosen to move wastewater effluent disinfection away from chemical (principally chlorine) based disinfection processes. Disinfection of final effluent by ultraviolet light irradiation has been a widely used alternative since the early 1990s. It is effective against bacterial, viral AND protozoan pathogens and leaves no chemical residual or byproducts in the effluent. Disinfection by UV irradiation relies on the UV light penetrating the effluent flow and applying a certain 'dose' of UV light to a unit of water and, consequently, to the pathogens in that unit of water. The effectiveness of that irradiation can be affected by particulate matter in the water, cloudy water conditions and the presence of dissolved organic matter. Conventional UV technology (low or medium pressure lamps) normally requires ultraviolet light transmissivity (UVT) of at least 40% for UV disinfection to be effective at reasonably achievable dose rates.

Supercritical UV disinfection (SCUV) is an emerging technology for wastewater effluent disinfection that aims to make feasible, the disinfection of wastewater effluent with characteristics of low UVT and or high TSS / high turbidity, conditions under which conventional UV technologies would struggle.

This paper describes the technology, its applicability, it's application and the results from early installations.

There is currently a single manufacturer (Novolabs Ltd) developing and manufacturing the technology. This paper has been prepared with their consent but aims to be independent of the manufacturer and their involvement has been restricted to the provision of factual information about the technology.

Supercritical UV, as an emerging disinfection technology was mentioned, briefly, in the WEFTEC 2024 Pre-Conference Workshop — 'Understanding and Applying Disinfection Fundamentals'. This paper provides more in depth consideration of the technology.

What is Supercritical UV?
Each UV module consists of a 2D array of 4 conventional 260W low pressure UV tubes, behind a quartz sheet (as opposed to quartz tubes in a conventional 3D array) above the flow.
Two power settings are available (depending upon the effluent quality and disinfection needs). The low power setting uses 2 lamps (520W/module) and the high power setting uses all 4 (1.04kW / module)
Flow is pumped into the system (unless site topography allows for a gravity feed from the process immediately upstream — i.e a sloping site) although the process itself is by gravity and open to atmosphere
The modules are sloped such that super critical flow is established, with a corresponding very thin sheet flow of 2-6mm depth, resulting in extremely short path lengths for incident UV light
The flow channel and the cover above the lamps are both lined with reflective surfaces to maximize the amount of light directed through the flow.
To date, installations have been containerized but it does not appear that this would have to be the case.

Applicability While currently installed and operational at a limited number of municipal and industrial facilities the concepts presented are applicable and transferable to any wastewater effluent. Particularly, this applies to those situations where pre-treatment to a high UVT and or low TSS quality is impractical and (for other reasons) unnecessary. This would seem to make it particularly applicable to Wastewater Stabilization Pond (WSP) / Lagoon based treatment systems where seasonal temperature and nutrient variations regularly result in algal blooms up to and beyond 200mg/l TSS. The EPA Lagoon Inventory Dataset indicates that there are some 4,500 Lagoon based WWTPs in the United States. To date, the current maximum installed SCUV capacity is 9 MLD (2.4 MGD). It is expected that this supercritical UV technology will be scaled beyond that capacity.

Results and Conclusions
Results from system deployment, to date and at full scale appear promising. Not least in addressing the problem of disinfecting high TSS, low UVT effluents without chlorine. Data and results will be provided from at least three full scale, permanent installations and two full scale demonstration trials.
A summary of results currently to hand is shown tabulated below.
Further actual operational data will be added up until paper submission time to augment that already available to produce the results above.

If these results can be achieved consistently and reliably, the implication, for very large numbers of small to medium sized treatment plants, particularly those based on waste stabilization ponds or with significant levels of dissolved organics (such as blood) is that:
Non-Chlorine based disinfection of those effluents will be feasible, and
UV disinfection stage precursor processes, in many instances, may be able to be avoided, resulting in capital and operational cost savings for what are, frequently, communities with very limited financial resources.