The biofilm method, like the activated sludge method, is a method of using microorganisms to remove organic matter from wastewater. The material that provides attachment, growth, and fixed surface for the biofilm is called a filler, which is an important factor affecting the development and performance of the biofilm method.

The basic principles of biofilm method

1. Formation and characteristics of biofilms

The biofilm method is a biological treatment method that uses bacteria and other microorganisms attached to the surface of a carrier or medium to grow and reproduce, forming a membrane like activated sludge - biofilm, which degrades organic matter in wastewater. Microorganisms in biofilms use organic pollutants in wastewater as nutrients, degrade organic matter during metabolism, and also proliferate themselves.

With the continuous reproduction and growth of microorganisms, as well as the continuous deposition of suspended solids and microorganisms in wastewater, the thickness of the biofilm continues to increase, resulting in changes in the structure of the biofilm.

In the biological treatment process, biofilm always grows, updates, and falls off continuously. The reasons for biofilm continuous falling off include hydraulic erosion, weight increase due to membrane thickening, loosening of protozoa, weak adhesion between anaerobic layer and medium, etc.

The biofilm method is suitable for the biological treatment of small and medium-sized sewage. The sewage treatment system can be independently established or combined with other sewage treatment processes. Before the biofilm treatment of sewage, it is advisable to undergo sedimentation treatment. When there are significant fluctuations in the inflow water quality or quantity, a regulating tank should be set up.

2. Structure of biofilm and its mechanism for purifying wastewater

Biofilm is a fluffy flocculent structure with numerous micropores, a large surface area, and strong adsorption capacity. Biofilm microorganisms use organic matter adsorbed and deposited on the membrane as nutrients, converting a portion of the material into cellular material for reproduction and growth, becoming new active substances in the biofilm. The other portion of the material is converted into excreta, releasing energy during the conversion process to meet the needs of microbial growth. After the proliferation of biofilm falls off, it enters the wastewater and is intercepted in the secondary sedimentation tank, becoming sludge. If

When the organic load is relatively high and the biofilm cannot oxidize and decompose the adsorbed organic matter in time, it can form unstable sludge, which needs to be reprocessed. Due to the presence of microorganisms in an attached state in the biofilm process, the sludge age is long, resulting in both short generation and high growth rate microorganisms in the biofilm, as well as long generation and low growth rate microorganisms. This results in more microbial species participating in metabolism in the biofilm process than in the activated sludge process.

3. Main characteristics of biofilm method

Compared with the activated sludge method, the biofilm method has the following characteristics:

⑴ Biofacies characteristics:

① Diversification of microorganisms involved in purification reactions

② The food chain length of organisms

③ Microorganisms that can survive for longer generations

④ Segmented operation and dominant species

⑵ Process characteristics：

① Strong resistance to impact loads

② Good sludge settling performance, suitable for solid-liquid separation

③ Capable of treating low concentration wastewater

④ Simple operation, energy-saving, easy maintenance and management, low power cost

⑤ Less sludge generated

⑥ Under low water temperature conditions, it can also maintain a certain degree of purification function

⑦ Has good nitrification and denitrification functions

The main influencing factors of biofilm method

There are many factors that affect the biofilm process, such as water quality, temperature, pH value, dissolved oxygen, nutrient balance, concentration of toxic and harmful substances, etc. These factors are also factors that affect the activated sludge process. As mentioned earlier, the following will introduce the unique influencing factors of the biofilm process.

1. Hydraulic load

The hydraulic load has a certain impact on the treatment effect of biofilm method, as well as the thickness and mass transfer improvement of biofilm. There are multiple treatment processes for biofilm, and an appropriate hydraulic load should be selected.

2. Surface structure and properties of the carrier

The impact of carriers on wastewater treatment efficiency is mainly reflected in the surface properties of carriers, including the specific surface area of carriers, hydrophilicity and surface charge of carriers, surface roughness, carrier density, porosity, and material strength. The choice of carrier not only determines the size of the area and biomass available for biofilm growth, but also affects the hydrodynamic state in the reactor.

3. Biofilm mass and activity

The thickness of biofilm reflects the size of biomass, but biological activity is not always positively correlated with biomass. The biofilm is composed of aerobic and anaerobic membranes, with a thickness of usually 1.5-2.0mm. The degradation of organic matter is mainly completed within the aerobic layer. We cannot simply pursue increasing the biomass of the reactor. We should ensure that the biofilm inside the reactor falls off and updates normally without the phenomenon of carrier gaps being blocked.

Types and process flow of biofilm method

There are various classifications of biofilm methods, which can be divided into fixed bed biofilm method and flowing bed biofilm method based on the presence status of the carrier attached by microorganisms. Fixed bed biofilm can be divided into biofilters and biological contact oxidation methods, while flow bed biofilm methods include fluidized beds and moving beds.

According to the degree of immersion of biofilm in wastewater, biofilm method can be divided into submerged biofilm method, semi submerged biofilm method, and non submerged biofilm method. Common submerged biofilm methods include biological contact oxidation tanks, aerated biofilters, etc. Common semi submerged biofilm methods include biological turntables, and common non submerged biofilm methods include biofilters. biofilters are further divided into three types: ordinary biofilters, high load biofilters, and tower biofilters.

1. Ordinary biofilter

⑴ Process flow

Ordinary biofilters, also known as drip filters, are the early types of biofilters that emerged, namely the first generation of biofilters. The sewage enters the initial sedimentation tank first to remove settleable suspended solids, and then enters the biofilter. The wastewater treated by the filter and the aging biofilm that falls off the biological filter material flow into the secondary sedimentation tank. After solid-liquid separation, the purified water is discharged.

⑵ Construction

A regular biological filter consists of four parts: the tank body, filter media, water distribution device, and drainage system.

① Pool body

Its flat form is often square, rectangular, or circular, and the pool walls are generally built with bricks, stones, or reinforced concrete.

② Filter material

The surface of the filter material has biofilm attachment, which is the main body for purifying sewage. The filter material has a significant impact on the working efficiency of the biofilter. Biological filters generally use solid fist shaped inorganic filter materials, such as crushed stones, pebbles, and slag. In recent years, plastic filter media have been widely used in biological filters, mainly processed from polyvinyl chloride, polyethylene, polystyrene, etc. into composite filter media such as corrugated plates, honeycomb tubes, rings, and hollow cylinders. Its characteristics are light weight, high strength, corrosion resistance, large specific surface area, and high porosity, greatly improving membrane growth and ventilation conditions, and greatly enhancing treatment capacity.

③ Water distribution device

Ordinary biological filters often use fixed nozzle water distribution systems, which mainly consist of siphon devices, water distribution tanks, water distribution pipelines, and nozzles.

④ Drainage system

The drainage system at the bottom of a regular biological filter is located below the filter material layer, mainly responsible for collecting and discharging treated wastewater, ensuring ventilation and supporting the filter material. The drainage system is usually divided into two layers, including a seepage device under the filter material and a collection and drainage ditch at the bottom plate.

⑶ Process characteristics

① Ordinary biological filters are generally suitable for treating small town wastewater and organic industrial wastewater with a daily sewage volume of no more than 1000m3. They have high purification efficiency, good treatment effect, and effluent quality

Stable water quality.

② Low infrastructure investment, stable operation, easy management, low power consumption, and energy savings.

③ The remaining sludge amount is small.

④ Due to its low load and large footprint, it is not suitable for treating large amounts of wastewater, and its flushing capacity is insufficient, which can easily cause biofilm accumulation and blockage in the filter material, thereby affecting the ventilation inside the filter. During operation, filter flies will be generated, and the hygiene conditions are poor. Therefore, its use is limited.