When you consider the equipment and capital scope, an ultraviolet (UV) disinfection systems seems so much more substantial than a simple chemical feed and contact arrangement, that over the long haul, a UV system would appear to be more expensive—much more, in fact. If a Life-Cycle Assessment (LCA)
is performed, however, and other less-obvious sustainability factors are considered, the choice is no longer so cut and dried.
Dr. Tapas K. Das
of St. Martin’s University
in Washington presented a detailed LCA comparison at the Annual Meeting of the AIChE in October, and guess what…?
Life-Cycle Assessment Rationale and Methodology
An LCA’s intent is to identify all
costs and effects of a project or system over its entire life. These include
monetary, societal, environmental, and opportunity costs and effects from project inception through final decommissioning and remediation required at the economic end of the project.
This includes “downstream” costs and effects that seem obvious, like chemical and biological impacts on the air, land, and water, for example. LCA effects also include “upstream” effects. Upstream effects are the various impacts resulting from the inputs
to a system. Thus, if a process utilizes dimethyl-doorknob (DMD) as a raw material, an LCA will factor in the economic, societal, environmental, and opportunity costs and effects of the manufacture and transportation of DMD. This process continues upstream to the inputs to the manufacture of DMD, and so on. Since use of DMD requires all of those effects to be realized, they are taken into account in the LCA.
UV Disinfection Systems
The principle of UV systems efficacy is the property of radiation of a wavelength around 270 nm to
penetrates into the cell and disrupt its DNA or RNA. Thus damaged, the organism can not reproduce. Wastewaters thus disinfected can be legally discharged to surface waters. There is no residual effect of UV, so in drinking water systems a chlorine/chloramine feed is also required.
For the 18 Million Gallon per Day (MGD) example used in this evaluation, a total of nearly $6 million of capital investment is required, centered around the lamp assemblies, light chambers, power supplies, and electrical installation. A large number of “kill chambers” are required, due to the limits of path length (how far the UV light can penetrate in the wastewater stream) and the “dose time” (how long the stream must be exposed. Operating costs are primarily electricity, lamp replacement, and preventative maintenance and cleaning. Upstream costs are limited ones associated with the manufacture of the equipment and ongoing ones associated with electrical power generation. Total annual operating costs of $380,000/year are estimated for this UV system.
Chemical Disinfection Options and Factors
Chlorination disinfection systems can be based upon using chlorine gas (Cl2
) or liquid bleach (NaOCl(aq)
) as the free chlorine source. In addition, either the free chlorine itself, or chloramines (upon reaction with ammonia compounds) can be the disinfection agent, with chloramines being more stable but less effective. In wastewater applications, any residual free chlorines need to be eliminated, most often with sodium
Chemical disinfection systems are much less capital-intensive and are comprised of gas or liquid chemical feed into simple reaction tanks, sized for the necessary “kill” residence times, and then subsequent dechlorination reaction time prior to discharge. Controls of feed are basic and usually based upon Oxidation-Reduction Potential (ORP) sensors. The high costs of these systems are a result of the chemicals required. There are significant costs and impacts in the manufacture, transportation, and storage of hazardous chemicals, in addition to significant compliance and safety costs. The potential environmental impacts of the chemicals, as well as their aquatic toxicity issues upon use, are also significant. This leads to much higher annual operating costs of $667,000-793,000/year.
And the Winner Is…
In the final life-cycle assessment performed on the UV and chemical disinfection options for an 18 MGD plant, the total costs were very similar. The UV plant “nosed out” the bleach option, and was itself nosed out by the chlorine option, with <0.4% separating them. The chemical systems were not so much less expensive after all.
In addition, some of the chemical systems' potential risks of toxicity impacts were unable to be cost quantified; thus, the UV system engendered a qualified safety and environmental advantage as well.
Contact Dr. Das
for further details of the work at St. Martins University.
Images by Dr. Tapas Das.