Containment and processing of odors, as well as emissions of VOCs (volatile organic compounds) and HAPs (hazardous air pollutants), whether regulatory-driven or good neighbor-/community-driven, are forcing municipal and industrial wastewater facilities to evaluate and re-formulate their processes so they can achieve the mandated or desired atmospheric emission levels. Obviously the choice of any specific method of containment will have a significant effect on the cost, performance and success of the odor control system installed.
The 1990 Clean Air Act specifies and lists the HAPs and VOCs and requires the USEPA to set maximum allowable emission standards for all major sources of emissions. To date, the most prominent regulations have been CFR 40 part 61 NESHAP (National Emission Standards for Hazardous Air Pollutants), with standards for specific industries and applications being continually issued and modified.
Compliance with the new regulations will affect the economics of wastewater treatment facilities in a substantial way. The impact will be in the form of
The types of covers over process units and the method of containment selected can have the most significant impact on these costs and will affect
Containment and downstream processing of the offending elements would appear to be the corrective path of choice. Not only is the technology to accomplish this available, in many cases it would allow for capacity increases in existing plants. Facilities that have been forced to lower throughputs and slow down aeration and other process rates to avoid or reduce undesirable emissions, potentially can reverse these actions through appropriate containment and process technology.
Various regulations affect the need, method and other aspects of containment of hazardous emissions from wastewater facilities. The Clean Air Act defines areas and processes that require containment measures; OSHA deals with safety-related issues of contained and covered areas. NFPA 820 deals with fire protection, explosion potential, and ventilation of covered and non-covered areas. These regulations must be incorporated into projects that consider containment alternatives.
Setting the goals for a wastewater emissions containment project must take into account the myriad of regulations that exist. They must be evaluated and balanced in a total sense, so that compliance with all, and not just some, is achieved. The relationships among economics, efficiency and successful plant operation also have to be weighed into the final solution.
While the containment system chosen may represent only a fraction of total project cost, the method of containment and the covers specified will significantly influence other components or factors. A successful design project will attain the following goals:
Factors Affecting the Selectionof a Containment System
Controlled ventilation and air flow rates are absolutely essential in a properly functioning emission and odor control system.
Large volumes of low pressure, constantly flowing air are required in wastewater treatment plants. Once volume and pressure requirements have been determined, the handling equipment can be selected after the air volumes, leakage rates and air exchange rates are established. Most emission sources in wastewater systems are treated by means of an actively ventilated system using a fan or blower to induce air movement over the process area, conveying the air either to other parts of the plant or through a process unit. This requires a constant low negative pressure. Typically, air is replaced so that flow rates and pressure can be regulated.
The source and method of air replacement is extremely important. Ideally, air is to be replaced through controlled ventilation points and vacuum relief openings. An effective active ventilation system must take into account the negative air pressure requirements of the emission source as well as the desired maximum VOC concentration within the system. Insufficient negative pressure may allow fugitive emissions to escape to atmosphere, as well as improper air exchanges that could lead to fire or explosion hazards. Improper air exchanges and dead air pockets will cause severe corrosion problems, particularly in the case of concrete. Dead air spaces cannot be allowed to occur or large chunks of concrete will be eroded away by high concentrations of sulfuric acid formed by hydrogen sulfide gas. Too high a negative pressure will cause excessive air flow and increases the cost of treating the emissions. Proper control of the air flow's volume and velocity are imperatives if the system is to run successfully.
Ideally, airflow should be maintained at the point where the maximum safe concentration of VOCs is delivered to a control device, and at the minimum negative pressure to produce an in-draft condition at all wastewater flow rates. Normally, either VOC concentration or the negative pressure requirements will dictate the design flow. If the air flow rate is set by the negative pressure, then sealing gaps and cracks is essential. If VOCs determine the air flow rate, then the system must be regulated to ensure that adequate air is drawn into the contained space to dilute the VOC concentrations. In any event, it is imperative that the system's air flow and tightness be regulated and controlled.
The size of the air handling equipment in wastewater applications can be reduced in the following ways:
NFPA 820 and OSHA 1910.146 concern themselves with the rate of air exchanges within these contained and covered areas, for the sake of air quality and breathing and asphyxiation considerations, as well as concentrations of explosive gases. Areas that require entry for routine equipment and process monitoring not only, by design, have a larger volumetric space, but require air to be exchanged a minimum of 12 to 16 times per hour. At the same time, areas not requiring entry will need four to six air exchanges per hour. As a rule, covered areas where entry is neither allowed nor needed will require air to be exchanged at a rate that is 50 to 75 percent lower than areas that must be entered for process control and maintenance activities.
Large volumes of air will result in additional one-time costs for larger air handling equipment (fans, blowers, scrubbers, ducts, etc.) along with heavier in-plant power transmission and switchgear requirements. Operational costs will be the greatest cost factor. The larger volumes of air will affect maintenance costs, as well as disposable air filtration products such as carbon and scrubber media. Since wastewater treatment facilities typically operate continuously around the clock and on a year-round basis, even the most modest gains in air moving efficiency can have significant financial benefits. A design combining lower volumes, a minimum number of air exchanges, a high degree of containment, and minimal air leakage clearly approaches the conditions required for the ideal cover and containment system.
Safety concerns should be carefully addressed. Covers, particularly flat covers, should be sturdy and not require railings or other accessories which might restrict access. Designers should provide access in the form of hatches, vents and ports. Flat covers should be lightweight and not exceed 150 lb per unit, within the 75 lb maximum lift for a single person as allowable under OSHA regulations, while not requiring special lifting equipment. Forward looking designers should limit cover weight to 100 lb per unit in anticipation of newly-contemplated OSHA workplace regulations limiting maximum allowable lifting weight to be 50 lb per worker. These requirements for strong lightweight clear-span covers should favor marine grade corrosion resistant aluminum as the material of construction.
Containment systems incorporating raised structures should be evaluated to determine if they might be defined as confined spaces. Once a space has been designated as confined, considerations other than air handling must be taken into account. One-time equipment costs in the form of, for instance, safety suits, portable air breathing apparatus, lights, alarms, gas and air quality monitoring equipment, and premiums paid for explosion-proof motors and junction boxes should be be factored into the initial purchase price. The added operational costs associated with confined spaces will far exceed any differential in initial equipment costs. Permitting and record keeping tasks, training and safety drills, and hole watch requirements are but three of the mandated functions that are bound to influence costs significantly. The requirement that an attendant be present outside each time an entry is made into the confined space is the costliest requirement of all. The advantages of dealing with a covered area that is not designated a confined space can amount to substantial operational savings that may dwarf the initial equipment cost.
Operation & Maintenance
While raised spaces, particularly domes, offer convenient access to some equipment, particularly center drives on basins with rotating arms of some type, they do have drawbacks. Scum wells, collection boxes and weirs may not be easily accessible. Apparatus not close to the center of the tank (particularly in circular domed applications) may not be very accessible from the central interior area of the structure. Hatches and side openings may have to be be provided, so that inspection and access are possible. Flat covers can provide access in the form of swing hatches directly above pieces of equipment. They also can lower maintenance costs by isolating bridges and railings from the corrosive environments frequently found inside the contained areas in wastewater treatment plants. On the occasions that drives and other mechanisms need to be attended to or removed, the dismantling and access provisions of a raised structure can be considerably costlier than in the case of flat covers. The removal of a dome may involve the services of contracted, specially trained factory personnel for removal and for reinstallation. On the other hand flat covers usually can be removed and reinstalled by plant workers for a relatively low cost.
Methods of Containment
Typically the least costly method of containment is a non-rigid system using film, membrane, or fabric placed over the offending area. Operationally, it is difficult to maintain a negative air pressure without causing the system to collapse. While it may be possible to achieve a high degree of tightness, the attainment of the required air circulation, however, is very difficult to achieve. This type of system is restrictive as far as access is concerned, if not outright dangerous. Since it cannot support any weight, loads caused by snow or rain, and certainly by plant personnel, the system must be considered a non-accessible, operationally restricted area. Furthermore, the removal and installation procedures required, as well as possible frequent repair of tears in the material, make the choice of such a system impractical in many designs.
Semi-rigid systems have been used in designs for covering areas where instruments and other components have to be supported, but they offer neither rigidity nor containment capability. Any type of planking in the form of corrugated light weight steel, aluminum, fiberglass or wood should be discouraged. Their use can result in a a dangerous platform that lacks good containment properties. These features, along with the higher operational costs that tend to be associated with non-tight installations, should be factored into the design and selection equation.
Raised rigid structures are commonly used in odor control installations and do offer high containment efficiencies. But their operating costs tend to be on the high side. They require bigger air handling components such as larger fans, blowers, ducts, and scrubbers, and heavier-duty electrical accessories and wiring. Greater outlays for electricity, maintenance needs, chemicals and neutralizing media follow. Raised structures offer good access for O & M tasks required for equipment located at the center of the contained area, but poor access if major work is called for. As a result of the large volumetric space, they contain atmospheres that can be excessively corrosive to equipment above the water level in the process basins covered, for example bridges, railings, motor housings etc. Raised structures cover in most cases what are defined as permit requiring confined space areas, and as such will be subject to additional one time safety-related equipment costs. Structures designated as confined spaces also will incur costs caused by the obligatory permitting, training, and hole watch regulatory requirements.
Flat covers offer safety features, reduced air volumes, good access through hatches and openings directly over equipment. Relatively low equipment costs are due to the smaller air volumes that result from the geometric configuration and the smaller number of required air exchanges. Flat covers also can eliminate the need for compliance with confined space regulations. Furthermore, they offer good access for major equipment maintenance and removal. However, they may lack air tightness. The inherent design of a system constructed of modular covers, limited by weight, and mandated to be easily removable, creates a structure that contains many cracks and gaps between its components. Flat covers are meant to be modular and their segments relatively small to comply with weight restrictions. They are not to be bolted to each other to facilitate easy removal. At the same time they are to provide for thermal contraction and expansion. These features result in a system that is less efficient.
It is for this reason that flat covers can be designated as typical flat covers and tight flat covers. The latter type carries the process an extra step, adding to the features described above the most important characteristic of tightness. Even the slightest gaps (1/8 in.) between covers-what may be acceptable commercial tolerance-will have a major negative effect on operating costs.
The insistence on tightness should not be waived, and claims such as "air tight" or "substantially air tight" should be investigated thoroughly. A properly designed system will have no air gaps, whether by design (or design deficiencies) or otherwise, as gaps cause significant reductions in its odor control performance. A well designed system should seal off these gaps by means of gaskets or other filler material, although caulking is not considered as an acceptable method of achieving tightness.
Basic steps should be taken to confirm that the design intent is indeed maintained, when the actual system is finally delivered and installed.
About the Author:
Arie Kepets is head of design and product development for Thermacon Enviro Systems in New York City.