All water treatment systems require a number of technologies to meet their effluent treatment goals. Remediation of a light non aqueous phase liquid (LNAPL) plume, for example, may require an oil-water separator, air stripper, and carbon adsorption system to meet discharge requirements. However, it is often wise to add other components to protect the primary treatment equipment, but not necessarily directed at the primary contaminants of concern.
For example, filtering solids upstream of granular activated carbon (GAC) units will prevent premature GAC fouling. Another example is installing a cone bottom inlet equalization/feed tank, rather than using a flat bottom tank. Settleable solids can be captured in the cone prior to further treatment. The cone bottom can be easily accessed to pump out the sludge without draining the entire tank.
Other protective equipment examples include:
- Upstream iron removal of oxidation technologies to reduce oxidation dose requirements.
- Clarification of semi-volatile organics prior to air strippers to reduce fouling.
- Knock-out pots upstream of oxidizers and vacuum blowers to remove moisture from the process air.
- Inorganics removal to protect membrane systems from fouling and scaling.
- Solvent removal to protect membrane systems.
- Grit removal to reduce pump impeller wear.
- Water softening to reducing scaling in wet scrubbers.
Materials of construction are another important consideration. Care should be taken to choose not only those which are compatible with water contaminants, but also where maintenance activities are likely to occur. For example, coating or painting mating surfaces of bag filter housings is not recommended. As these housings are accessed to change baskets and bags, painted surfaces will chip and crack. Though stainless steel construction may represent larger initial capital cost, the equipment will require less lifetime service.
General equipment layouts should also take into account regular maintenance activities. Pipe runs should not be located across access hatches, and adequate clearance must be given to fully access the trays in low profile air strippers. Large basket strainers must be located high enough from the ground that the baskets can be removed. Instruments which need to be regularly calibrated (such as pH sensors) should not be located in elevated duct or pipe runs.
Adequate space around commonly maintained areas (pumps, blowers, actuators, belts) should be allowed, if possible, to ensure operators have their boots on the ground and are not working in tight conditions.
Deposits, both organic and inorganic, can cause either premature equipment replacement or major maintenance costs to restore full functionality.
Inorganic deposits are well known. Hardness scale and iron deposition are the two most common culprits. However, there are other equally problematic but more industry specific one, such as struvite precipitation in landfill leachate systems. Biofouling from bacterial growth can also quickly gum up a system but is often disregarded in the initial design process. This is either an oversight or because the influent waters are not characterized properly beforehand. Fouling can also occur from the process stream itself, especially when the water contains fat, oil, or grease in significant quantities.
Inorganic deposit control can be handled in a number of ways. Metals can be intentionally removed, substituted (as in softening), or sequestered so they remain in suspension. The method chosen depends heavily on flow rate, residence time in the system, and discharge requirements. For example, if the discharge permit has expressed limits for iron or calcium, then removal may be required. If not, sequestration may be a better option, allowing the metals to pass through the system, protecting the equipment, and lowering maintenance costs.
Organic deposit control is often overlooked since the potential for biofouling is not commonly characterized during the design phase, especially for pump and treat systems. However, bacterial growth can clog bag filters, foul carbon systems, encumber pipes, and blind off membranes.
To control bio growth, operators can either disinfect or discourage bacterial growth by removing environmental conditions which would promote growth (e.g., removing food sources, or adjusting redox potential). Disinfection can be accomplished by the addition of chlorine, sodium hypochlorite, chlorine dioxide, biocides, and UV radiation, to name a few. However, each of these systems brings unique operator challenges, especially in terms of chemical handling, health and safety, and disinfection byproducts.
Disinfection should occur as soon as possible in the system to provide maximum protection. It can also be used to shock a system back into compliance when bio growth is out of control.
Long-term management, as well as removal of bio scum, can also be accomplished by injecting bio dispersants, which can remove food sources, weaken cell walls, and inhibit bacterial reproduction. Most bio dispersants are typically safer to handle than biocides or corrosive chemicals, and do not react with the treatment system itself.
Lastly, operators should have a preventative maintenance plan in place on the first day of operation and should be aware of all the manufacturer’s maintenance recommendations and warranty exclusions. This includes a complete schedule of mechanical, electrical, and controls checks.
Mechanically speaking, all moving parts need to be checked for wear and tear, pump alignments should be verified, seals and gaskets should be checked for integrity, leaking tanks identified, and all process equipment checked for inorganic and organic deposits.
Electrically speaking, all control signals should be verified, and control panel components should be verified for correct operation, including all system switches and control buttons.
Finally, the control system should be checked to make sure that all alarms are operating correctly (high-or-low level switches trip properly), the sequence of operations is still valid, and all instruments are properly calibrated.
Performing regular and routine maintenance keeps small problems from becoming maintenance nightmares and inflating operation and maintenance budgets beyond acceptable levels.
The operations and maintenance costs for large remediation systems often eclipse the initial capital expenditures. Poor attention to design details can turn a routine maintenance schedule into an oppressive task.
During a site visit, one of Anguil’s preventative maintenance engineers observed that a large pipe header had been plumbed across the front face of a low-profile air stripper, used to access the removable trays. Removing the dirty trays for cleaning and replacement with clean trays, normally a 30-minute job by a single operator, now required two days of work and two operators to safely remove and re-plumb the header.
This problem could have easily been avoided during design or installation phases by either rotating the air stripper or rerouting piping.
Another example was the location of a pH sensor in an elevated pipe run three metres from the ground. A simple three-point recalibration of this sensor now required the rental of a man-lift. A second operator was needed to read the calibration values off the control panel, since a local LCD display was not provided on the pH analyzer.
Many such maintenance headaches could have been avoided if an experienced operator and installation expert had been consulted when the system was still just on paper.
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