When it comes to sewage treatment plants, people's first reaction may be that they have a strong odor. Indeed, during the treatment process of sewage, whether it is the grille, sedimentation tank, biochemical tank, or sludge dewatering room, a foul odor will emerge that makes people frown - it contains hydrogen sulfide (smelling like rotten eggs), ammonia (pungent fishy smell), and various volatile organic compounds, which not only affect the lives of surrounding residents, but also are not good for the health of factory workers. So, odor treatment design is definitely an "invisible key project" in the construction of sewage treatment plants. Today, I will talk to you in plain language about how to do this.
Firstly, it must be clarified that odor treatment is not a "one size fits all" approach, and one cannot simply install a set of equipment without considering the situation. The first step must be to "understand the situation", that is, to investigate and test the concentration of odor pollution sources. This is like a doctor "asking questions" before seeing a doctor, knowing where the disease is and how serious it is, in order to prescribe the right medicine.
How to investigate specifically? You have to follow the treatment process of the sewage plant for a circle. For example, in the front grille, as soon as the sewage enters, the organic matter inside begins to decompose and the odor first comes out; Then there is the sedimentation tank, where impurities carried by the sand and gravel ferment, and there may also be an odor; A biochemical pool is a place where microorganisms decompose pollutants. When microorganisms work, they produce a lot of gases, and the concentration of odors is often highest here; There is also a sludge dewatering room, where a large amount of odor is released during the sludge squeezing process, and because the space is relatively enclosed, the odor is more likely to accumulate.
After investigating the source of pollution, the next step is to measure how strong the odor is. We cannot rely solely on our nose to smell, we need to use professional equipment to measure the concentrations of major pollutants such as hydrogen sulfide and ammonia, as well as the "dimensionless concentration" of odors (simply put, the degree of odor). For example, the concentration of hydrogen sulfide in the grille may be 5-10mg/m ³, while in the biochemical tank it may be 20-50mg/m ³, with significant differences in data across different regions. Only by obtaining these data can we have a basis for selecting equipment and making plans later. Otherwise, designing out of thin air will either result in insufficient processing effectiveness or wasted money.
After understanding the situation, the core step is to design the odor collection system. Many people think that "processing" is the most important, but in fact, if "collection" is not done well, even the most powerful equipment in the future will be useless - the equipment here is working hard to process it, while the foul odor runs out from the cracks, which is equivalent to useless work.
The key to collecting a system is to "cover up" and "take it away". How to 'cover up'? The collection method should be selected based on the shape and working conditions of different structures. For example, places with fixed equipment such as grilles and dehydration rooms are suitable for using "partially enclosed covers", like putting a transparent "hat" on the equipment to trap odors in small spaces; For large-scale, open structures like biochemical tanks, they need to be covered with a "sealed lid", such as a fiberglass cover plate or a flexible tarpaulin. It is important to leave an inspection opening on the cover plate, otherwise it will be difficult to maintain the equipment in the future.
The 'extraction' relies on ventilation ducts and fans. The pipeline design has its own requirements, and it cannot be done simply by pulling a pipe. Firstly, the diameter of the pipe must be accurately calculated and determined based on the amount of odor emissions in each area. If the diameter is too small, it will cause too fast wind speed, easy pipeline wear, and noise; If the pipe diameter is too large, it will waste materials, and if the wind speed is too slow, the odor may still accumulate and condense in the pipeline. Secondly, the pipeline should have a slope, usually a slope of 1% -3%, to prevent the condensation of water vapor in the odor into water, which can accumulate in the pipe and block the road, and also corrode the pipeline. In addition, the air volume of the fan also needs to be matched to ensure that there is a "negative pressure" in each enclosed space - in simple terms, the air pressure inside is lower than outside, so that fresh air from outside will not enter, and the odor inside will not escape, but will only be extracted by the fan to treat the equipment.
After collecting the odor, it is time to enter the "processing stage", which is the key to determining whether the odor can be removed. There are various treatment technologies on the market now, and there is no absolute best one. Only the "most suitable" one needs to be selected based on the previously measured odor concentration, pollutant type, as well as the factory's budget and land occupation size. Let's pick a few of the most commonly used ones to chat with.
The first one is the biofilter method, which is currently one of the most widely used technologies in sewage treatment plants, with the advantages of being "environmentally friendly and cost-effective". The principle is particularly interesting, which is to let the odor pass through a pool filled with fillers (such as tree bark, volcanic rock, peat soil). The fillers are attached with many microorganisms that specialize in "eating odor" - these microorganisms treat pollutants such as hydrogen sulfide and ammonia as "food", and after digestion, they become harmless water, carbon dioxide, and nitrogen.
When designing a biological filter, there are several points to pay attention to. Firstly, the selection of fillers is crucial. It is not advisable to simply use soil piles, but rather to choose materials with high porosity and good water retention, such as tree bark mixed with volcanic rock. High porosity is essential for the smooth passage of odors, while good water retention is necessary for the survival of microorganisms (which require a moist environment). Next is the height of the filter, usually 1.5-2 meters is sufficient. If it is too high, the resistance will be high and the fan will have to consume more electricity; If it's too low, the processing effect won't be enough. Also, before entering the filter, the odor needs to be "pre treated" - cooled, humidified, and if there is dust in the odor, it needs to be removed. Because microorganisms are afraid of high temperatures (they cannot survive above 40 ℃) and dryness, excessive dust can clog the pores of the filler.
The second method is chemical absorption, which is suitable for situations where the odor concentration is relatively high and the pollutant composition is complex, such as the high concentration odor coming out of a biochemical pool. The principle is to allow the odor and chemical agents (such as sodium hydroxide and sodium hypochlorite solution) to fully contact in the absorption tower, and the agents and odor substances undergo chemical reactions, turning them into harmless substances.
The design focus of this method is on "sufficient exposure". Absorption towers are generally selected as "packing towers", which are filled with plastic packing materials. The chemicals are sprayed from the top of the tower, and the odor rises from the bottom of the tower, so that the gas-liquid can be fully mixed on the surface of the packing materials. The concentration and dosage of the medication need to be accurately calculated. For example, when treating hydrogen sulfide with sodium hydroxide solution, a concentration of 5% -10% is generally sufficient. If the concentration is too high, it will be wasteful, and if it is too low, it will not be thoroughly treated. In addition, a "demister" needs to be added behind the absorption tower to prevent chemical droplets from being discharged along with the treated gas, causing secondary pollution.
The third method is activated carbon adsorption, which is suitable for treating low concentration and difficult to degrade volatile organic compounds. It is commonly used as a "deep treatment" - for example, after being treated with a biological filter, if there is still a little residual odor, it can be adsorbed with activated carbon to meet the emission standards. The principle is simple. Activated carbon has many small pores on its surface, which act like a "sponge" to absorb odor molecules.
When designing an activated carbon adsorption tower, attention should be paid to the replacement cycle of activated carbon. Do not wait until the activated carbon is "fully absorbed" before replacing it, otherwise it will be ineffective. Generally, based on the concentration of odor and the amount of treatment, it is estimated that it will be replaced every 3-6 months. In addition, activated carbon is afraid of water, so the odor must be dehydrated before entering the adsorption tower, otherwise the water vapor will block the small holes of activated carbon, affecting the adsorption effect.
Finally, there is another easily overlooked point: exhaust pipe design. The processed gas must be discharged through an exhaust pipe, which cannot be too short, otherwise the processed gas will float back into the factory or surrounding residential areas. Generally, the height of the exhaust pipe is required to be no less than 15 meters, and if there are high-rise buildings in the surrounding area, it needs to be appropriately raised. At the same time, it is best to install an online monitoring device on the exhaust pipe to monitor the concentration of emitted gases in real time. In case of exceeding the standard, problems can be detected and equipment can be adjusted in a timely manner.
In general, the design of odor treatment in sewage plants is a "systematic project", from preliminary investigation and testing, to collection systems, and to the selection of treatment technologies, each step must be based on the actual situation, and cannot simply copy others' plans. Only by considering every detail can we truly solve the problem of "smelly" sewage treatment plants, which can not only treat sewage without affecting the surrounding environment, but also achieve "environmental standards and neighborhood harmony".
