Water & wastewater

Air flow measurement is required in aerobic digesters and aeration basins. In digesters, oxygenation allows microbes to decompose complex organic compounds. Concrete aeration basins utilize fine or coarse bubble diffusers to aerate the water. In addition to digesters and aeration basins, air flow monitoring may also include air going to an aerated grit tank, to a flow equalization tank, or to secondary sludge holding tanks. Challenge Aeration blower systems must supply a wide range of airflows with a relatively narrow pressure range under varied conditions. In addition to detecting the presence or absence of air flow, the flow switch should be able to detect low-flow conditions.

Carbon is employed in adsorption processes for the removal of organic, inorganic, and taste and odor control compounds. Granular Activated Carbon (GAC) and Powder Activated Carbon (PAC) are two leading adsorption treatments, the former utilizing a carbon bed and the latter employing a contact basin. Carbon slurry tanks require level monitoring. Challenge Handling and safety problems are associated with the fly-away dust and difficult-to-wet characteristics of carbon. As carbon buildup often chokes mechanical level controls, non-contact monitoring technologies are popular. Contact level sensing should use single rod probes to avoid media buildup.

In small wastewater treatment facilities with only one clarifier, it is advantageous to know the clarifier’s surface level. In larger plants where influent flow is routed through a splitter box to many clarifiers, there may be insufficient room inside the splitter box to accommodate level instrumentation. In this instance level monitoring of the clarifier will help attain proper load balance. Challenge Continuous monitoring of the clarifier level will maximize efficient treatment flow within primary and secondary sedimentation areas where the majority of suspended solids are removed.

In the anaerobic decomposition process, organic matter is converted into stable solids and energy-rich gas, mainly methane (60%) and carbon dioxide (40%). A combustible hydrocarbon of high fuel value, methane can serve as a fuel for in-plant heat exchangers, blower engines or other systems, marketed to the local industrial community, or burned off. Challenge Safe and reliable flow measurement is essential in the collection, disposal or re-use of methane gas. Because methane is highly combustible, flow instrumentation must be certified for operation in these hazardous locations.

The principal biological methods employed in wastewater treatment for sludge stabilization are aerobic and anaerobic digestion. The former utilizes aerobic bacteria to convert organic matter and stabilize biosolids while the latter accomplishes this goal with anaerobic bacteria working without oxygen. Both processes are monitored for level within their tanks. Challenge The gas-injection and mechanical stirring systems of anaerobic process, and the aeration systems of aerobic process, create large amounts of foam that have traditionally hampered mechanical level controls. Recent developments in radar technologies have advanced measurement accuracy despite the foam content of these digesters.

Following the clarification phase of water treatment, pre-filtered water passes from an inlet channel and onto a filtration bed. As it passes through the bed’s media—sand or anthracite in depth-filtration types; cloth or synthetic membranes in surface-filtration types—the water is cleansed of its fine-grained suspended solids. Challenge Filter tank level triggers the backwash cycle. As solids accumulate within the filter, headloss begins to build up and water level increases in the filter tank. A pre-determined tank level indicates that terminal headloss value has been reached, and that the filter must now be backwashed to remove the suspended solids.

Flow measurement is accomplished by a transmitter programmed to convert a level reading into units of volume per time, as liquid passes through a flume or weir in an open channel. A flume is a specially shaped portion of the open channel, with an area or slope that is different from the channel’s slope or area. A weir resembles a dam placed across an open channel positioned so that liquid can flow over it. Challenge Measurement devices are configured according to the geometry of the flume or weir. Devices must provide reliable measurement despite turbulence, solids content, corrosive chemicals, or varying flow velocities and flow depths. Environmental requirements often mandate the use of a totalizer and data logger.

Pre-treated wastewater is brought up from inlet trunk sewers to ground level by centrifugal pumps, where it continues on by gravity flow to subsequent treatment. To maintain a desired level in the headworks wetwell, pump speed is often varied. Pumps turn on and off automatically according to an operator selectable sequence. Challenge A desired wetwell level reduces the drop downstream of the influent flumes to minimize turbulence and air entrainment, and eliminates frequent pump cycling associated with variable level control. Proper level control maintenance in the wells can alleviate pump surging, which disrupts settling by causing currents and eddies in the clarifier.

In terms of annual tonnage, lime ranks first among water treatment chemicals. Lime is used for pH adjustment, phosphate removal, sludge and biosolids conditioning, and in association with other chemicals for precipitation. Lime is initially mixed with water in lime slakers to form a slurry referred to as slake lime. The slake lime is then added to raw water for formulation under carefully controlled conditions. Challenge Often contained in an agitated tank, lime slurry is slightly abrasive, and can be corrosive depending upon the other chemicals involved. Contact level sensing should use single rod probes to avoid media buildup.

There are several areas within a wastewater treatment facility where solids settle and are subsequently removed as sludge. These include hoppers, holding tanks, gravity thickeners, and other sludge collection systems. Sludge level is monitored to control against incomplete discharge or dilution of the sludge. Challenge Non-contact measurement is often preferred since sensors are located above the process fluid and beyond direct contact with the sludge. Sensors of contact technologies should be single rod types to minimize material buildup – or bridging – that occurs in twin rods configurations.

Wastewater treatment facilities with a large number of primary clarifiers often employ a concrete tank with chambers and gates known as a splitter box, or distribution box, to split the influent flow into multiple streams that are routed into the bank of clarifiers. A similar device may also be used to split mixed liquor flows between secondary clarifiers. Challenge Level control in the splitter boxes helps maintain balanced routing of influent wastewater in the primary sedimentation process where 50 to 70 percent of the suspended solids are removed, and in the secondary sedimentation process where remaining suspended solids are removed.

Service water is utilized for general plant services that include pump and instrument seal water, fire water, demineralization, cooling and make-up water supply. Storage tanks with a capacity to support three days to one week of operation, allow continued plant operations in the event the supply of water is interrupted. Collectors and storage tanks are typically fixed roof, vertical cylindrical steel tanks. Challenge Level measurement and flow detection devices are crucial for effective water source management. Typical measurement ranges are from 24 to 50 feet.

Storage for reclaimed water ranges from small tanks for potable and service water use, to large, open or enclosed, reservoirs. Large capacity storage – most commonly in open reservoirs – is necessary for off-line storage of peak flows, flow-through in-line storage, and long term storage of seasonal flows that are discharged during alternate seasons. Challenge Level monitoring is essential for large and small water storage tanks. Controls are specified according to the size and geometry of the bulk storage vessel. Level controls in open atmosphere reservoirs must frequently withstand punishing weather conditions.

The compressor of a gas turbine ingests a large amount of air containing particulate matter, aerosols of hydrocarbons and other organic compounds and gases. Although the larger particulate matter is filtered out, the other compounds are deposited on the compressor blades. Compressor washing removes this deposited fouling and restores the aerodynamic profile and compressor efficiency. Also used for cleaning generator or other machinery and equipment components, water wash is periodically discharged as waste water. Challenge Water wash is collected in a dedicated collection tank monitored for level with typical capacities of 50 to 100 gallons.