Sewage handling: selection and cleaning of MBR membranes, calculation of membrane tanks!

What is MBR?

MBR, also known as membrane bioreactor, is a new water treatment technology that combines activated sludge process with membrane separation technology. There are currently four main types of MBR membranes: hollow fiber membranes, flat membranes, ceramic membranes, and tubular membranes.

Hollow fiber membrane (also known as curtain membrane) and flat membrane are collectively referred to as "organic membrane", mainly made of PVDF (polyvinylidene fluoride), which is the main material for all membranes on the market today. But now there is a new PTFE (polytetrafluoroethylene) material, which is said to be better than PVDF. When pulling the film, the pore size distribution will be more uniform, known as the "plastic king".

Ceramic membranes have also emerged in recent years, and together with organic flat membranes, they are called "flat membranes". Ceramic membranes have excellent chemical stability and can withstand acid, alkali, organic solvents, and high temperatures; The advantages of narrow pore size distribution and high separation efficiency. The main materials of the ceramic film I am exposed to are aluminum oxide, titanium oxide, and it seems that there is also silicon oxide.

Organic tubular membrane, as a form of membrane element, is suitable for membrane separation technologies such as ultrafiltration, microfiltration, and even nanofiltration. Its advantages are wide flow channels, turbulent flow of feed liquid in the tube, and low requirements for pre-treatment accuracy of feed liquid. The tubular membrane is easy to clean, and in addition to being cleaned with chemical reagents, it can also be mechanically and physically scrubbed. The pressure loss of the tubular membrane module is small, so its flow channel is long (up to 48 meters in series) and the filtration efficiency is high.

A flat film is a plate (usually ABS) with a guide channel on top, and a film made of lining cloth and PVDF material is pasted on both sides. After seamless welding, it forms a film. There are also two types of membranes, also known as flat films, one is soft film, and the other is flexible film. Flat membrane component: A common type is a frame made of 304 stainless steel welded together. The membrane is inserted into it one by one for fixation, and the aeration pipe fittings and collection pipe fittings are installed, which is called a membrane component. The commonly used range of 10-250 pieces can form components as integers, regardless of the size of a single film. In theory, if the diaphragm is damaged and the water production decreases, it can be inspected and replaced on a single piece, thereby saving replacement costs.

How to select membrane components?

Choose the membrane flux based on water quality, 10-26L/m2h. For example, for domestic sewage, 18-22L/m2h can be selected, and for electroplating, pharmaceutical and other wastewater, 12.5-15L/m2h can be selected for calculation. Also, pay attention to the composition of the treated water entering the membrane tank, whether there is a large amount of oil, calcium and magnesium ions, and organic solvents that can blend with the membrane, etc.

Membrane pool calculation formula:

Number of membranes n=Q ÷ N ÷ Ax1000

Q: Daily processing water volume t/d

N: The flux of the membrane L/m2d (the selection of flux has already been mentioned, it can be changed to days, try to take the lowest value)

A: The effective area of each membrane (commonly ranging from 1.5 and 0.8 square meters, but also from 2, 1.6, 1.0, and 0.9 square meters)

Because the selection of membrane area is related to the construction of the membrane tank, the depth of the membrane tank needs to be determined first. For an effective water depth of 3-3.5 meters, a membrane of 1.5 square meters can be selected, and for an effective water depth of 2.2-2.5 meters, a membrane of 0.8-1.0 meters can be selected. For general civil engineering pools, consider 1.5, and for steel box pools, consider 0.8.

The selection of membrane pool is determined by the effective water depth, the size of the selected membrane, the length of the pool, and the width of the pool.

Taking a flat film of about 1.5 square meters as an example, the length is approximately between 1600-1800mm, the width is between 470-550mm, and the thickness is between 5-7.5mm. Almost the same.

Taking a 100t/d domestic sewage as an example, calculate the number of membranes:

According to the formula: n=100 ÷ 400 ÷ 0.8x1000=31.5 (pieces) is not an integer, so it needs to be rounded. Because the minimum value of 400L/m2d has already been taken, 300 pieces can be directly taken here. 100 pieces per component require 3 components, and 150 pieces per component require 2 components. Choose based on the size of the membrane pool.

The required membrane pool sizes for these two types are:

Three components: 4 * 2.5 * 2.8m or 3.5 * 2.5 * 2.8m

Two components: 3 * 3.2 * 2.8m

The length of the membrane pool is equal to the number of components plus one meter, which ensures even placement of the components. Compression can be performed when there are many components, but not when there are few. Consider installation.

The width of the membrane tank, considering the placement of components, the routing of the aeration and drainage systems, as well as the convenience of installation and maintenance by workers, should provide a relatively spacious space. The common component width is calculated based on 11-14 millimeters per film, with 200 millimeters reserved for each end of the frame, and 350 millimeters reserved for the installation of water collection and aeration. Width required for 100 films: 100x14+400+700=2500 millimeters

How to clean the membrane components?

The operating pressure of the flat membrane is 0-35KPa. The mixed liquid in the membrane tank is pumped through a micro porous filter membrane made of polymer materials under the action of a self suction pump (10 minutes per working cycle, 8 minutes for pumping and 2 minutes for stopping). Its uniform pore size is used to intercept particles, bacteria, etc. in the water, so that they cannot be removed through the filter membrane. This filters the sludge containing wastewater in the biochemical reaction tank and achieves sludge water separation. So, the flat membrane plays a filtering role:

1. The membrane intercepts microorganisms in the reaction tank, causing a significant increase in the concentration of activated sludge in the tank (commonly around 6000-12000mg/L), making the biochemical reaction of wastewater degradation more rapid and thorough;

2. Due to the high filtration accuracy of the membrane, the effluent is ensured to be clear and transparent, resulting in high-quality water production. So let me explain here that membranes are not omnipotent. They have pore sizes, and even those that can pass through them cannot be filtered. Don't rely solely on this stage for the hope of improving standards. What cannot be filtered will form fouling and blockage near the membrane pores, which is what we call membrane blockage. Insufficient water production and insufficient flux cannot be achieved.

When it comes to membrane blockage, let's talk about the cleaning of membrane components. Cleaning mainly includes online cleaning and offline cleaning

Here we mainly talk about online cleaning. Generally, online chemical cleaning is used for cleaning, and the cleaning cycle depends on the pollution of the membrane. For example, domestic sewage with better water quality may be cleaned every 3 to 4 months, while for leachate with poor water quality, it needs to be cleaned every 15 or 20 days. The specific judgment is based on the water yield and negative pressure. The common cleaning method for solution preparation is to first wash with alkali and then acid wash according to the situation, and there may also be cases of acid wash before alkali wash.

Alkali washing is mainly a mixed aqueous solution of sodium hypochlorite and sodium hydroxide. Acid washing often involves oxalic acid or citric acid. If there are many calcium ions, try not to use oxalic acid. This different manufacturer will provide corresponding parameters, which are basically 3-5 L of solution per film. Adding them up will calculate how much is needed. Medication can be divided into gravity flow or dosing pump insertion..

Simple operation for cleaning:

When the membrane is slightly polluted during operation, online cleaning can be used, which is simple to operate, cost-effective, and has a significant effect; Generally, alkaline washing is carried out first, followed by acid washing. Gravity flow or dosing pump can be used for dosing.

Alkali washing: Prepare a mixed aqueous solution of 2000-5000mg/L sodium hypochlorite and 1000mg/L sodium hydroxide, and use a high liquid level cleaning water tank to fill the suction pipeline to the membrane element under static water pressure. The dosage of the solution is 3-5L/tablet, and soak for more than 5 hours.

Pickling: Prepare a 1000mg/L oxalic acid (or citric acid) solution, clean online with the same amount of water, and soak for 3-5 hours.

Note: The above reagents are calculated based on 100% purity. When the selected reagent contains impurities, the corresponding dosage is calculated based on the corresponding purity. The method of using gravity self flow of the medicine liquid to inject the medicine liquid into the membrane through the reserved dosing port on the membrane suction tube for cleaning the membrane.

Offline cleaning, also known as maintenance cleaning, is generally performed once a year or when online cleaning cannot restore membrane flux. Set up an offline cleaning tank or membrane tank for cleaning, which means soaking the entire tank with chemicals and restoring flux through aeration and flushing.