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Addressing TSS in secondary effluent to improve UV disinifection
With increasing risk management requirements and the desire to avoid multiple chemical feed systems, municipalities are looking to utilize ultraviolet (UV) disinfection systems at wastewater treatment plants (WWTPs) as an alternative to conventional chlorine systems. In many cases, UV disinfection can carry a lower cost of operation than chlorine-based systems if the system size, design and operations are opti- mized. One of the most important factors in pro- viding efficient wastewater UV disinfection is the effluent total suspended solids (TSS).
When designing UV disinfection systems, UV transmittance (UVT) at a wavelength of 254 nm often is used as a rule of thumb to identify the fea- sibility of this method. In actuality, TSS often have greater impact on UV performance, as shown by samples collected from a conventional WWTP at two different times (Figure 1). Collimated beam studies, which are laboratory tests to determine the relationship between UV dose and bacterial inactivation (response), were conducted and showed a nearly full log of inactivation improvement for the sample with lower TSS, although UVT was 5% higher.
Thus, improving TSS is critical for improving UV disinfection efficiency—first, because an increase in TSS can decrease the disinfection rate, especially at lower UV doses, and second, because tailing of the dose-response curve can occur (Figure 2). Tailing generally does not occur until low concentrations of bacteria have been reached; as a result, it often is the primary limiting factor to reaching regulatory standards. Poor UV disinfection attributable to solids is one of the primary reasons that tertiary filtration is assumed to be required for wastewater UV disinfection. But there are hundreds of instances when UV has been successfully implemented for secondary effluent.
In designing these systems, the concentration TSS typically is used to evaluate potential UV performance. It also is important to consider that, when larger particles are present, bacterial shield- ing can result in lower disinfection rates. Particles larger than 11 to 12 μ begin to affect disinfection; particles larger than 20 μ are crucial in protecting particle associated bacteria. As a result, UV doses required to inactivate microorganisms associated with particulates require two, three or more times higher doses to achieve the same inactivation as “free” microorganisms. To improve UV performance, there are two particularly noteworthy strategies:
1. Physically remove particles, e.g., tertiary filtration, addressing particle associated bacteria by reducing bacterial concentrations to the UV system. But, because some particle shielding can be overcome by increas- ing the UV dose, disinfection of unfiltered wastewater effluent with TSS concentrations up to 30 mg/L has become quite common. In addition, high capital costs associated with tertiary filtration has sometimes been used to justify higher design doses, which in turn requires additional equipment and energy.
2. Modify operation of upstream processes to reduce the number and size of TSS particles. While this approach is viable and well documented, it is not widely implemented.
Designing the Right UV System
During the feasibility evaluation of UV dis- infection, particle size analyses and collimated beam testing should be conducted, in addition to the review of other effluent data. While these tests represent a single operating condition, results provide insight into the level of concern with regard to bacterial shielding. If TSS concentrations frequently exceed 30 mg/L, a significant fraction of particles fall in the above-20-μ size range or if collimated beam results indicate dose response tailing, then the design engineer will need to address TSS using one of the strategies outlined above.
In conducting the disinfection system evaluation, capital and operations costs for the alternative systems should be compared. These also should include consideration of tertiary filtration if that process is deemed necessary. For UV disinfection to be competitive with alternatives, it is important to size the systems appropriately. Many engineers and UV equipment manufacturers have databases on particle size for a range of biological processes. For example, data from four different WWTPs demonstrate how different upstream processes can impact particle size distribution (Figure 3). TSS concentration is provided in the figure legend.
In Figure 4, it may be noted that the distribution of particle sizes in trickling filter (TF) plants can be variable, which is why UV disinfection often is considered for these applications. Operation of TF facilities can result in periodic sloughing of biosolids that may not settle well; so while the effluent may be amenable to UV disinfection most of the time, episodic sloughing events can cause permit violations. There are options such as tertiary filtration, ballasted flocculation and other enhanced clarification processes to remove solids; however, there are more economical means of improving solids without the need of expensive tertiary treatment.
Upstream Process Optimization
The Massard WWTP in Barling, Ark., is a prime example of how upstream process optimization can be implemented to allow efficient UV disinfection. A disinfection study was conducted for the facility, originally constructed with primary clarification, trickling filters, secondary clarification and a chlorine contact basin. Chlorination-dechlorination alternatives and UV disinfection were evaluated using life-cycle costs, showing that UV could be cost-effective and reliable if the upstream processes could be upgraded and operated to consistently produce effluent with a UVT greater than 55% and average TSS less than 20 mg/L.
Process upgrades included the implementation of enhanced biological flocculation, which is gen- erally attributed to several genera of heterotrophic bacteria. The mechanism of bioflocculation is the result of bridging between extracellular microbial polymers that cause cells and groups of cells to stick together. In practice, this includes the addition of a short detention time to promote the growth of the appropriate organisms. Once constructed, the improvements resulted in improved TSS removal efficiency (Figure 4), which allowed successful conversion of the existing chlorine contact basin to UV disinfection.
Results are consistent with work at other facilities and demonstrate how effluent TSS characteristics can be improved so UV disinfection of secondary treated effluent is optimized. Upstream process control strategies often are more economical than physical removal particles using tertiary treatment.
Additionally, operational changes, such as small increases in solids retention time, also can reduce effluent bacterial concentrations exponentially. These examples provide evidence that enhanced biological flocculation is an economical method to aid in particle coagulation and settling prior to disinfection, potentially eliminating the need for expensive tertiary processes that may make UV disinfection less competitive with other disinfection technologies.