The purpose of tertiary treatment is to provide a final treatment stage to raise the effluent quality before it is discharged to the receiving environment (sea, river, lake, ground, etc.). More than one tertiary treatment process may be used at any treatment plant. It is also called “effluent polishing.”
“Tertiary Filtration” systems must include a physical filtration process designed to achieve an effluent quality of 10 parts per million biological oxygen demand and 10 parts per million suspended solids.
The use of filtration systems for waste water systems is relatively recent development. Concept of utilizing filtration in waste water treatment came into practice due to stringent requirements for secondary reuse. The low levels of SS & BOD were difficult to meet with conventional secondary treatment. Tertiary filtration is aimed at removing the fine suspended solids that are carried over with effluent in secondary clarifier. The BOD associated with the suspended solids also automatically gets removed during filtration resulting into low SS/BOD effluent.
A typical Granular Media Filter used for Tertiary treatment
Granular media filtration systems remove fine non-settleable material. Media systems include silica sand, anthracite, GAC, gravel, garnet all available in mono, dual, and multimedia form. Under drain systems are manufactured in plenum and lateral styles, incorporating slotted dome strainer nozzles.
Backwash systems are available as manual and automatic control and comprise air scour, combined air scour/low rate backwash, low rate backwash and high rate backwash phases as appropriate. Backwash principles are discussed in detail later.
Filter designs available include:
– Conventional open gravity cell
– Pressure filters
– Automatic self backwashing filters
– Filter rate control methods include level controlled, rising level and declining rate.
Filter media systems are designed to suit the application and include:
– Mono sand media
– Coarse deep bed media
– Dual media (coal/sand)
– Multimedia (coal/sand/garnet)
As like regular filters, the tertiary filters too need regular backwash. In fact tertiary filter need better backwashing techniques as the load on them is higher and also there is possibility of mudball formation.
Mudballs are agglomerations of extraneous material which accumulates in the bed over extended periods of time due to improper backwashing. Consequently the effectiveness of filtration is greatly affected.
Cleaning filters is necessary to remove the solids collected by the media during the filtration run. As solids are accumulated in the filter media, the headloss or force required to maintain the flow increases. A point is reached where either the flow cannot be maintained or solids are driven through the filter.
There are three common methods of cleaning filters in use today: hydraulic backwash only, hydraulic backwash plus sub-surface wash, and hydraulic backwash plus air scour.
The traditional method of cleaning a filter has been to reverse the flow and bring clean water up from beneath the bottom of the bed at a rate sufficient to fluidize the media and shear off the floc.
Chemicals and trapped solids can adhere tightly to filter media grains. The cleaning of granular filters by the upward flow of backwash water alone to fluidize the filter bed is inherently a weak cleaning method because it is very sensitive to flow and the shearing action may not be adequate to remove chemical floc from the media. If the media is not completely cleaned each time, dirt can accumulate causing mudballs. If mudballs are allowed to accumulate, they can sink to the bottom of the media and plug the gravel. The bed will then become upset, resulting in poor performance and loss of media.
Hydraulic Backwash Plus Surface Wash:
Another problem encountered in filters is the build-up of a crust of solids on the surface of the media. Chemicals and dirt can attach to the upper surface of the media and form a crust which may be too heavy to fluidize and backwash off. If it is not removed, this crust can form mudballs and ruin the filters.
The fact that a high rate backwash is not enough to eliminate mudballs has been recognized for a long time. Mudball formation is especially prevalent with highly turbid waters. Under these conditions some kind of auxiliary scour is considered necessary.
A common practice is to install a distributor to clean the media surface. This distributor, or “surface wash” mechanism, is situated just below, approximately four inches below the media. Nozzles distribute high-pressure water to scour the media surface as well as provide motion to the distributor.
The surface washer is normally turned on for 1 or 2 minutes to scour the surface. Then the backwash flow is begun, expanding the media up past the surface washer and allowing it to continue its scouring action deeper in the bed.
Although sub-surface wash is a substantial improvement in cleaning a filter, it does not completely eliminate mudballs. Nor does it clean the entire bed. The circular agitator has difficulty cleaning the corners where mudballs can form.
Hydraulic Backwash Plus Air Scour:
The current practice worldwide is to provide an air scour in place of the surface wash. Air scour in the range of 30 -48 m3/m2/hr provides much more violent agitation of the media than surface wash. In addition, air scour combined with a low rate backwash (concurrent air and water) is the most effective way to remove solids from a filter. The media grains have a greater potential to collide, increasing the scrubbing action during the air scour. Side-by-side tests have shown that air scour is the most effective backwash method and uses the least water.