Wastewater Treatment Knowledge: Meaning and Function of the Biofilm Process
The biofilm process (biomembrane process), also known as the attached-film process.
Structure of the biofilm on biofilter media (cross-section)
● A class of aerobic biological wastewater treatment technology that ranks alongside the activated-sludge process; it is an attached-film method and an artificial, intensified version of the soil self-purification process.
● Mainly removes dissolved and colloidal organic pollutants from wastewater.
● Main categories: biofilters include the conventional biofilter, the high-rate biofilter, and the tower biofilter.
Aerobic biological fluidized bed, etc.
The biofilm process is a method of treating organic wastewater using microorganisms (i.e., a biofilm) attached to the surface of certain solids. The biofilm is an ecosystem of densely packed aerobic bacteria, anaerobic bacteria, facultative bacteria, fungi, protozoa, and algae; the solid medium to which it attaches is called the filter medium or carrier. From the medium outward, the biofilm is divided into an anaerobic layer, an aerobic layer, an attached-water layer, and a flowing-water layer. The principle: the biofilm first adsorbs organic matter in the attached-water layer and the aerobic bacteria in the aerobic layer decompose it, then it enters the anaerobic layer for anaerobic decomposition, while the flowing-water layer washes away the aged biofilm so a new one grows—repeating to purify the wastewater.
A general term for biological treatment methods in which microorganisms grow along the surface of a solid (called a carrier). So named because the microbial community grows into a mucous membrane along the solid surface. When wastewater contacts the biofilm, pollutants transfer from the water onto the membrane and are thus treated. Its basic mechanism is described under biological wastewater treatment.
A typical biofilm-process flow. The bioreactor in the flow (Fig. 1) can be a biofilter, a rotating biological contactor, a biological aerated filter, or an anaerobic biofilter. The first three are for aerobic processes, the last for anaerobic. The earliest biofilm bioreactors were intermittent sand filters and contact filters (tanks filled with broken pieces). Both ran intermittently—filter–rest, or fill–contact–drain–rest—forming a cycle. They evolved from sewage irrigation and are based on soil self-purification. Then continuous biofilters appeared. After new plastics emerged, further development followed.
The most commonly used biofilm bioreactor. The carrier is small pieces (e.g., gravel, plastic packing) or plastic blocks stacked into a filter bed, hence often called filter media. Unlike ordinary filters in water treatment, the biofilter bed is exposed to air and wastewater is sprayed onto it. Distributors come in fixed and moving forms; the rotary distributor is most widely used. It consists of two or more symmetrically arranged horizontal perforated pipes that rotate about the tank center; the pipes sit close to the bed surface and water flows from the holes. The distributor works continuously, but watering of local bed areas is intermittent, inheriting the intermittent-irrigation concept of sewage irrigation. Below the bed is a collection layer of brick, special ceramic blocks, or concrete; below that the tank bottom. The collection layer connects to the outside, draining and ventilating. In operation, wastewater flows down the carrier surface through the bed, exchanging matter intimately with the abundant microorganisms and attached water on the carrier. Pollutants enter the biofilm; metabolites enter the flow. Effluent carries sloughed biofilm debris and needs a settling tank. The biofilm gets dissolved oxygen directly or via the flow from air. In conventional biofilters the slime layer is thick and the part near the carrier is often anaerobic. Bed depth relates to filtration rate and media. Gravel beds were mostly about 1.8–2 m deep for a long time; deeper beds clog and pond at the surface. The filtration rate is about 1–4 m³/(m²·day); higher rates also pond the surface. The first breakthrough was raising the rate. When the hydraulic load (i.e., filtration rate) exceeds 8–10 m³/(m²·day), flow scouring prevents clogging and the organic (BOD₅) load can rise from about 0.2 to above 1 kg/(m³·day). To meet hydraulic load, influent is often diluted by reflux. For stable efficiency, two stages in series can be used. This innovated flow with higher load but unchanged structure is the high-rate biofilter. It was then found that raising bed depth from about 2 m to above 8 m improves ventilation; even at higher hydraulic load the bed no longer clogs and works well, and the organic load can reach about 1 kg/(m³·day). Because its plan diameter is about 1/6–1/8 of its height, tower-shaped, it is called a tower biofilter. Since plastic blocks appeared, ventilation and clogging are no longer problems, and bed depth and rate can be designed as needed.
Appeared with the popularization of plastics. Dozens to nearly a hundred plastic or FRP discs are strung on a shaft and laid in a trough with a semicircular cross-section. Disc diameter is usually under 4 m; trough diameter is a few cm larger. A motor and reducer turn the shaft at about 1.5–3 rpm depending on disc diameter; disc peripheral speed is about 15 m/min.
Wastewater flows from one end of the trough to the other. The shaft is above the water; about 40% of the disc is submerged and about 60% exposed to air. As the shaft turns, the disc surface alternately contacts wastewater and air. Covered by a microbial film, the biofilm alternately contacts wastewater and air fully, continuously taking up pollutants and oxygen to purify the water. Shear stress between film and disc grows with film thickness from rotation; beyond a point the film sloughs off and leaves with the flow.
Compared with biofilters, the rotating biological contactor gives longer wastewater–biofilm contact time and some controllability. The trough is often divided into sections and discs into groups, preventing short-circuiting and helping raise load and effluent quality as load drops stage by stage. If odor occurs, a cover can be added. RBC is generally used for small flows.
An aeration tank with plastic blocks, also called the biological contact oxidation process. It works like the aeration tank in activated-sludge but needs no sludge return, and aeration cannot be the same—generally whole-tank bubble aeration is used; biomass is far higher than in activated sludge, so aeration time can be shortened. Operation is stable with no sludge-bulking problem. Granular media (e.g., sand, activated carbon) are also used; flow goes upward, the bed expands and does not clog. High surface area, high biomass, and full contact shorten aeration and raise efficiency; still at the research stage.
Similar in structure to the biological aerated filter but without the aeration system. Because of high biomass, treatment time is much shorter than a sludge digester (retention >10 days), and it may be used for low-concentration wastewater such as municipal sewage.
● Prerequisites: a supporting carrier—packing or filter medium
● Nutrients—organic matter, N, P, and others
● Biofilm formation by inoculation: sewage containing nutrients and inoculated microorganisms flows over the packing surface; after some time microorganisms attach, proliferate, and grow into a thin biofilm.
On the biofilm, the ecosystem of bacteria and other microorganisms and the biofilm's organic-degradation function reach balance and stability.
For municipal sewage at 20°C, biofilm maturation from formation generally takes about 30 days.
Property: highly hydrophilic, with an attached-water layer
③ Becomes an aged biofilm with poorer purification function and tends to slough.
① As biofilm thickness grows, the inner depths where oxygen cannot penetrate turn anaerobic
② A mature biofilm generally consists of an anaerobic and an aerobic film
③ The aerobic film is the main site of organic degradation, generally about 2 mm thick.
① Increased anaerobic metabolites disrupt the balance between anaerobic and aerobic films
② Continuous escape of gaseous products weakens the biofilm's attachment to the medium
Highly dense microorganisms: various bacteria and microfauna that mainly remove organic pollutants from wastewater, forming a food chain of organic pollutant → bacteria → protozoa (metazoa)
① Aged film sloughs and new biofilm grows
② New biofilm has stronger purification function.
① Slow the biofilm aging process
② Control the anaerobic film thickness
④ Try to prevent concentrated biofilm sloughing.
(1) Strong adaptability to changes in flow, quality, and temperature; (2) good treatment with good nitrification; (3) low sludge (about 3/4 of activated sludge) and easy solid–liquid separation; (4) low power cost.
a. Using the biological aerated filter and biological contact oxidation tank of the biofilm process gave good results at the Xiufeng Industrial City wastewater station; BOD₅, CODcr, and suspended-solids removal were generally above 80%. The biological aerated filter performed slightly better than the contact oxidation tank.
b. Biomass and microbial amount in the biological aerated filter were markedly higher than in the contact oxidation tank—sludge mass concentration about double and microbial density 1–2 orders of magnitude higher.
In a wastewater structure, a carrier for microbial growth (generally packing) is placed; under aeration, microorganisms attach to the packing surface and form a biofilm. As aerated sewage (aeration by water-treatment blower and diffuser) flows at a certain velocity over the packing, biofilm microorganisms absorb and decompose the organic matter, purifying the water while proliferating and thickening the film. When the film reaches a certain thickness, oxygen diffusion into it is limited; the surface stays aerobic while the inner layer becomes anoxic or even anaerobic, eventually causing sloughing. Then new biofilm grows on the packing, repeating the cycle and purifying the water.
After microorganisms attach and form a biofilm, adsorption creates a thin water layer on its surface; the organics there have been oxidized by the biofilm, so its concentration is much lower than the influent. As wastewater flows over the biofilm surface, organics transfer from the moving wastewater into the attached water layer and are further adsorbed by the biofilm, while oxygen from the air also passes through the wastewater into the biofilm water layer and moves inward.
Biofilm microorganisms decompose organics and metabolize in the presence of dissolved oxygen; the resulting inorganic products such as CO₂ then move in the opposite direction—from the biofilm through the attached water layer into the flowing wastewater or air. Thus effluent organic content drops and the wastewater is purified.
For small-scale decentralized wastewater treatment, the biofilm process is advantageously used over activated sludge, specifically: ① Microbial phase—biofilm processes have diverse purifying microorganisms, longer food chains, and easy survival of long-generation-time microbes, with dominant strains forming in each stage of staged operation; ② Process—biofilm processes adapt strongly to quality/flow changes, have good sludge settleability and easy solid–liquid separation, can treat low-concentration wastewater, and are easy to maintain and energy-saving.
Classified by biofilm–wastewater contact mode:
Packed type includes biofilters and rotating biological contactors
Immersed type includes contact oxidation and biological fluidized bed
