First Things First

Aug. 11, 2020
Practical notes and useful guidance for pretreatment and primary stages of industrial wastewater treatment

Typically, treatment of industrial wastewater begins with primary treatment using processes such as screening or settling. Primary treatment may comprise clarifiers, American Petroleum Institute (API) oil-water separators, parallel plate separators, hydro-cyclone oil separators, or similar.

Subsequently, wastewater is biologically treated either under aerobic or anaerobic conditions (or both). These are known as secondary treatment.

Tertiary treatment is a set of final filtration(s), polishing and disinfection steps. The focus of this paper is on pretreatment and primary stages for industrial wastewater treatment. Practical notes and useful guidelines are presented.

Industrial Wastewater

Water clarity is affected by turbidity, which may be caused by inorganic (fixed suspended solids or FSS) or organic particulates suspended (volatile suspended solids or VSS) in the water. The latter may undergo biodegradation and thereby also have oxidation effects. Turbidity reduces light penetration.

Settleable particulates may accumulate on the bed of any waterbody, tank, vessel or eventually clarifier forming sludge layers. As the sludge layers accumulate, they may eventually become sludge banks and if the material in these is organic then its decomposition would give rise to odors. In contrast to the settleable material, particulates lighter than water eventually float to the surface and form a scum layer. The latter also interferes with the passage of light and oxygen dissolution.

Many industrial wastewaters contain oil and grease. This is particularly applicable to all oil, gas, refinery, petroleum, petrochemical and many chemical plants, although this can be experienced in a wide range of industrial facilities. Lubricating oils and hydraulic oils are also common contaminants in many industries. Notwithstanding oil’s organic or mineral nature, both types cause interference at the air-water interface and inhibit the transfer of oxygen.

Unlike domestic sewage, industrial discharges can have temperatures substantially above ambient temperatures (or sometimes below it). Apart from solubility of oxygen, rapid changes in temperature may result in thermal shock, which can have a wide range of effects.

Screens

Mechanical screens are usually used as the first step of treatment to separate relatively coarse solid and debris. Commonly used screens for large treatment plants are those with inclined bars, which should usually be of the multi-rake type bar screens. The removed solids and debris are discharged on the downstream side of the screen to a screening washer compactor or a container (when the compactor is out of service). The mechanical screens are manufactured to allow the screen to be pivoted out of the channel for maintenance and service.

Multiple identical screen units are often used for large plants. Mechanical screens are usually designed to remove materials that are larger than a certain limit. As an indication, this limit could be somewhere between 4 mm and 9 mm; 6 mm is commonly specified and used. Flowing velocity should be between a minimum velocity limit and a maximum velocity limit. As rough indications, minimum flowing velocity and maximum flowing velocity can be considered 0.5 m/s and 1.2 m/s, respectively. Normal flowing velocity should be around 0.6 m/s to 0.8 m/s. Low head loss type screens are encouraged. Ideally the head loss through the screen is below 0.4 m; typically, 0.45 m or 0.5 m are considered as maximum, although lower head loss (e.g., around or below 0.3 m) are always preferred.

Screen blockage is an important parameter often set between 20 percent and 25 percent. On meeting a blockage, the mechanical screen should be able to automatically reverse the direction of travel of the raking mechanism for an adjustable distance and revert to the forward motion to try and clear the blockage. This reversing action should be capable of occurring a minimum of three times for any one obstruction. The device should reset automatically if the blockage causing the initial overload condition is cleared. Screening bins are often sized to store 4 or 5 days' worth of screening storage, although some very large plants have specified a shorter period (one or two days) and some others have specified a longer period, sometimes even a week.

Equalization System

The objective of the equalization system is to minimize or reduce the fluctuations caused due to either sudden change of flow or composition (contaminations) in the wastewater treatment plant. Flow equalization provides dampening of the flow variations, thereby reducing potential spikes in flow and loads to the downstream treatment units. It also reduces the size of the downstream units and the cost of the overall wastewater treatment plant. The contamination/composition equalization provides dampening of contaminants, thereby preventing the shock loading of the downstream units such as biological treatment systems. In bio-treatment, performance is limited by the capacity of the microorganisms to adapt to the changing conditions of variation in flow and composition.

Equalization systems come in different types and concepts. The most common type of equalization system is the equalization tank. As an indication, the equalization tank is usually sized to absorb and store the difference between peak flow and average flow over a certain time period (e.g., the expected peak duration). As a very rough indication, this time period could be somewhere between 4 and 10 hours.

Primary Treatment

Primary treatment for industrial wastewater is usually a physical operation, most often gravity separation, to remove the floating and the settleable materials in the industrial wastewater. In a typical wastewater treatment plant, the primary treatment step consists of a clarifier system (primary clarifier) or an oil/water separator where oil, water and solids are separated. This is often followed by a secondary oil and solids separation step in which a dissolved air flotation (DAF) or similar unit is used.

Clarifiers

Clarifiers are well-known and well-established options for primary treatment. Clarification is the oldest and most widely used operation in the effective treatment of industrial wastewaters. The operation consists of removing sediment, turbidity, and floating material from industrial wastewaters.

There have been different types of clarifiers for wastewater treatment. The two commonly used types of wastewater clarifiers are the circular mechanical type and the parallel plate type. A circular center feed clarifier operates with the effluent entering through the center stilling well with the flow being forced downward. This ensures the proper residence time of the wastewater in the clarifier to allow for the settling of solids. The water then rises and exits through a wall-mounted weir trough that is placed on the inner circumference of the clarifier. A skimmer sweeps over the surface of the clarifier to collect any floatable solids and removes them via the scum trough. A scraper arm assembly passes over the bottom of the clarifier to densify and condition the settled solids (sludge) prior to being drawn off for additional processing. The sludge collection and handling systems need great care as there have been many complaints about these systems.

API Separators

Wastewaters from large-scale industries such as oil refineries, petrochemical plants, chemical plants, and natural gas processing plants commonly contain gross amounts of oil and suspended solids. Those industries use a device known as an API oil-water separator, which is provided specifically to separate the oil and suspended solids from their wastewater effluents. The name is derived from the fact that such separators are designed according to American Petroleum Institute (API) standard.

An API separator is a gravity separation device designed by using “Stokes Law” to define the rise velocity of oil droplets based on their density and size. This is based on the specific gravity difference between the oil and the wastewater because that difference is much smaller than the specific gravity difference between the suspended solids and water. The suspended solids settle to the bottom of the separator as a sediment layer, the oil rises to top of the separator, and the cleansed wastewater is the middle layer between the oil layer and the solids.

Typically, the oil layer is skimmed off and subsequently reprocessed or disposed of, and the bottom sediment layer is removed by a chain and flight scraper (or similar device). The water layer is sent for further treatment.

Although API separators have been widely used and well-accepted, there are still some performance-limiting factors. Emulsified or dissolved oil that is usually present cannot be removed by an API separator. These, therefore, should be treated in a subsequent treatment stage. Relatively high pH at the API separators can stabilize emulsions. It is desired to reduce pH at API separators for better treatment.

Skimming Devices

Many oils can be recovered from open water surfaces by skimming devices. Considered a dependable and cheap way to remove oil, grease, and other hydrocarbons from water, oil skimmers can sometimes achieve the desired level of oil separation. At other times, skimming is also a cost-efficient method to remove most of the oil before using membrane filters, chemical processes, or other methods. Skimmers will prevent filters from blinding prematurely and keep chemical costs down because there is less oil to process.

Because grease skimming involves higher viscosity hydrocarbons, skimmers should be equipped with heaters powerful enough to keep grease fluid for discharge. If floating grease forms into solid clumps or mats, a spray bar, aerator or mechanical apparatus can be used to facilitate removal. However, hydraulic oils and the majority of oils that have degraded to any extent will also have a soluble or emulsified component that will require further treatment to eliminate. Dissolving or emulsifying oil using surfactants or solvents usually exacerbates the problem rather than solving it, producing wastewater that is more difficult to treat.

Parallel Plate Separator

Parallel plate separators are similar to API separators but they include tilted parallel plate assemblies (also known as parallel packs). The parallel plates provide more surface for suspended oil droplets to coalesce into larger globules. Such separators still depend upon the specific gravity between the suspended oil and the wastewater. However, parallel plates theoretically enhance the degree of oil-water separation. Typically, a parallel plate separator requires significantly less space than a conventional API separator to achieve the same degree of separation. However, while parallel plate separators are often effective as two-phase separators (oil and water), they are less effective when a third phase (solids) is present. The solids that are present in wastewaters tend to foul and plug the parallel plates, resulting in the need for frequent maintenance.

Hydro-cyclone Separators

With hydro-cyclone separators, wastewater enters the cyclone chamber and is spun under extreme centrifugal forces up to 1,000 times the force of gravity. This force causes the water and oil droplets to separate. The separated oil is discharged from one end of the cyclone; treated water is discharged through the opposite end for further treatment. However, hydro-cyclone separators are not commonly used unless for specific plants. WT

About the Author

Amin Almasi is a lead mechanical engineer in Australia. He is a chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical) and IMechE (CEng MIMechE), in addition to an M.Sc. and B.Sc. in mechanical engineering and RPEQ (Registered Professional Engineer in Queensland). He specializes in mechanical equipment and machineries including pumps, condition monitoring, and reliability, as well as power generation, water treatment, and others. Almasi is an active member of Engineers Australia, IMechE, ASME, and SPE. He has authored more than 150 papers and articles dealing with rotating equipment, condition monitoring, power generation, water treatment, material handling and reliability. He can be reached at [email protected].