TRICKLING FILTERS: PROCESS FUNDAMENTALS
With apologies to colloidal matter, undesirable/excess wastewater constituents may be present in roughly either or both of two forms, namely soluble and/or particulate, a cutting point being about 0.45 µm. To a certain extent, nonsoluble fractions can or ought to be previously removed by appealing to essentially physical pretreatment means, e.g. settling, flotation, screening. This first step however, leaves us with an at times sizable amount of material in soluble form that will still have to be dealt with. Because further appeals to physical processes will be basically of no avail (or at least will behave as per diminishing returns laws), present environmental engineering practice resorts to and banks on the phenomenal power of natural/biological processes. Wastewater treatment plants essentially replicate in a somewhat controlled mode what has been going on in nature for ages: biological processes. In this way, “troublesome” soluble components are gladly gobbled up by living matter in specially conditioned “microorganism farms.” Typical embodiments include attached-biomass type biotowers and suspended-growth activated sludge basins or extended aeration systems. Common to all approaches is growth and development of large microorganism inventories that will pick up soluble stream components, be it organic matter, nutrients like N and P, which in turn can and will be subsequently removed from said stream by … physical means, e.g. settling, flotation, screening/membranes.
A trickling filter is basically a housing in which wastewater trickles through a bed of slime covered media and is treated by the action of the microorganisms in the slime layer, utilizing the contents of the wastewater as a food source. Trickling filter media or packing provide the surface over which microorganisms grow. Crushed stone or plastic are the most commonly used materials. From time to time dislodging or washing off of slime from the trickling filter media occurs, something commonly referred to as sloughing. Anaerobic activity at the slime media boundary is thought to be the mechanism responsible for this periodic biological growth stripping or wash out.
Biological filtration is thus a process of passing a liquid through a bed of appropriate media thus permitting contact with characteristically smooth, gelatinous biofilms attached to the media that assimilate fine suspended, colloidal and dissolved solids, and release end products of biochemical oxidation. Please be aware that contact time is relatively short albeit thus the need to insure large inventories of filter microorganisms. However as excess growth is not necessarily well/easily removed in unduly loaded trickling filters, clogging can and will occur.
TRICKLING FILTER DISTRIBUTORS
Fixed and/or rotary distributors are employed to uniformly apply wastewater (near-laminar flow) over trickling filter media. In the case of rotary distributors, flow is introduced at the center hub and manifolded to the arms. Rotary distributors can be hydraulically or electrically driven. Most typical configurations include either two or four arm arrangements. Bed diameters can go up to say 250 feet or 76m served with a single rotating distributor. Fixed distributors are basically pipe grids equipped with application suitable nozzles.
TRICKLING FILTER MEDIA
Structured packing as currently utilized for trickling filters can be categorized as cross-flute or crossflow media, and vertical-flute or vertical flow media .
Cross-flute or crossflow packing, proffering 31 sq.ft. per cu.ft surface area, is typically employed for trickling filters, including nitrification.
Vertical-flute or vertical flow packing, proffering 31 or 40 sq.ft. per cu.ft surface area, is typically employed for applications where media plugging can be a concern. However, vertical-flute media is not able to redistribute wastewater within each pack and only neglibly from layer to layer.
It is possible to combine both packing types in order to try get the best of each world, a concept and actual arrangement referred to as mixed media. One could/can aim at stacking the two upper filter media layers with higher reaction rate/higher performance cross-flute packing and utilize less fouling prone, vertical flute packing for all the other layers. The two upper layers would experiment the higher flushing rates which would tend to keep them clean. At the same time, the upper cross-flute packs will aid in uniformingly spreading or feeding incoming wastewater to the vertical-flute packs below.
Structured media designs can stack up layers of increasingly lighter modules thus reducing costs. Standard random packing, proffering uniform bearing capacity throughout the column height may result, to a certain extent, in otherwise avoidable wastage. One may thus opt for random media for the shallower say 3 m media depth and using modular media for larger depth filters.
Aside from failure to achieve intended removal rates, improperly designed trickling filters may give rise to odors, filter flies, clogging and media collapse.