Mannion, R., Evergreen Engineering Ltd, Ireland(free)
Microscreens also known as Rotating Belt Filters (RBF) are a keystone technology in wastewater treatment with multiple advantages improving the operation of wastewater facilities. These advantages are especially noteworthy in facilities utilising Biological Nutrient Removal (BNR) and include improved treatment efficiency compared to conventional primary clarification, smaller footprint, biological augmentation with improved nitrification rates, reduced odour, lower total CAPEX and lower overall OPEX. The prevalence of microscreen use is growing in the UK and throughout the world, with hundreds of plants taking advantage of the benefits of this technology. A detailed evaluation of this technology is useful to consultants considering facility upgrades and microscreen design options that will have a more positive lifecycle benefit compared to alternatives. This evaluation, how microscreens compare to conventional primary treatment, and the associated impacts on secondary treatment are presented in this paper.
Keywords: Eco MAT, Grit Removal, Headworks, Microscreen, Primary Filter, Primary Treatment, RBF, Screen.
An effective equivalent to screening and primary clarification, microscreens also known as Rotating Belt Filters (RBF), enable a treatment design in a minimal form factor with significant capital and operating costs savings. Ancillary benefits effecting economics of sludge management, augmentation of secondary biological treatment, and reduction of carbon footprint at wastewater reclamation facilities have been observed and measured in several cases throughout the world (Rusten & Ødegaard 2006).
Screening is important with all biological treatment systems. To advanced media systems, such as an IFAS processes, biological contactors, or membrane plants, effective screening is critical. Grit, rags, and hair are extremely detrimental to the vessels and features of these treatment systems. Grit chambers and bar screens certainly help, but often come up short in preventing irreversible fouling and damage to media and membrane modules. A more effective solution is necessary, and microscreens certainly have the capability to further alleviate the conditions leading to failure in biological media systems (Rusten & Lundar 2006).
The prevailing drivers for rethinking conventional settling and clarification include the massive footprint of settling, the cost of excavation and concrete works, reliability and quality of treatment, and the power demands for operations. Upgrades, including the expansion of design flow to the facility or process redesign to incorporate biological nutrient removal (BNR) are examples of situations that would benefit from a more foot-print-friendly technology. Availability of land is a factor that if considered alone can often make or break the feasibility of wastewater system upgrades or expansions (Nussbaum 2006). Many plant expansions are halted, slowed, or come with astronomical costs due to lack of space. Conventional sedimentation is a simple design but is land hungry, inflexible, and costly to install.
Several studies have emphatically demonstrated that primary treatment is not merely an option for wastewater treatment sites, but comes with defined benefits to the downstream biological plant (Razafimanantsoa 2014a & b; Paulsrud 2014; Jimenez 2014). The positive environmental impact potential is shown in all models to date, and is usually accompanied by an economical benefit as well.
Keeping in mind the limitations of conventional the primary treatment clarifier there are many technical and commercial reasons that engineers consider before implementation of microscreens. The microscreen require 5 percent of the footprint of a conventional clarifier and offers higher and more customizable levels of primary treatment. Not only can microscreens meet the treatment capacity in a smaller footprint as is apparent in Figure1, but N+1 redundancy with conventional clarifiers can certainly balloon project cost whereas with the microscreens redundancy is more economically feasible. Figure1 illustrates the relative footprint savings of microscreen technology when compared to conventional primary clarification of the same performance capacity; the rendition illustrates a nominal 12,000 m3/day treatment capacity footprint.
Figure 1: Microscreens are less than 5% of the footprint of conventional clarifiers. (please see paper)
Design Engineers often recommend microscreens as the site upgrade solution to screening and primary clarification at wastewater treatment facilities. The situations for implementation of microscreens are varied and include retrofitting treatment works that have no primary treatment works, storm water overflow, re-allocating primary sedimentation footprint to expand secondary treatment. On designing a new treatment works where space is a limited resource engineers have selected microscreens got primary treatment. The microscreen can be easily integrated on the treatment plant to gain treatment capacity by reducing the COD/BOD on the biological process. The microscreen strategy is being considered and applied throughout the Europe, the Americas, the South Pacific, and Asia.
Designing microscreens as primary treatment in new plants will save capital expenditure in equipment and civil works. The compact footprint of microscreens leads to potential savings in engineering, excavation, concrete, piping, scheduling, and many other aspects of the capital project. In many cases the capital studies shown microscreens to be between 25% of the conventional primary clarifier install cost. Where redundancy requirements are assessed there are significant commercial savings. Case histories also reveals that the operating cost is lower due to overall lower energy input. Even without consideration of the ancillary benefits, basic lifecycle cost analysis shows the microscreens beating conventional screening and clarification on direct costs alone.
Studies on ancillary economic impact are in progress to assess the total economic impact of microscreens. These studies include analysing the cost/benefit of the more effective COD removal, hair and grit capture inherent to the microscreens. An aspect of treatment with RBFs is that the technology provides a physical rather than hydraulic sequestration of particulate. The presences of hair in the secondary can create operational havoc in the secondary process. A range of activated sludge and fixed film secondary systems benefit from the mitigation of hair, as do membrane systems in particular. Reducing or eliminating the costly replacement of membrane modules or of media modules within the secondary and tertiary treatment systems appear having the potential of eliminating up to 50% of the annual OPEX for certain types of wastewater treatment subsystems, i.e. membranes. Multiple membrane manufacturers around the world are transitioning to RBFs for primary and pretreatment in membrane plants to extend membrane life.
Multiple engineering firms around the world have had an opportunity to study the umbrella of primary treatment technologies. Use of microscreens as an accepted solution for primary treatment is accelerating globally. There are several distinct challenges in screening and clarifying wastewater in municipal and industrial applications for which microscreens provide a ready answer, and their prevalence is particularly strong in Norway and Scandinavia countries. Over the past two decades microscreens have been erected on every major continent. With hundreds of plants around the world utilizing this technology, it is worthwhile to take a closer look at the design considerations.
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