We can all capture a concept of cure or medicine which can also be poisonous. It's worth noting that pretty much anything can be lethal in large enough doses. This also applies to chemicals used in sewage treatment.

Back in the day, the majority of sewage plants were classified as grade B in terms of emission standards. The use of biological treatment was considered to be enough to meet the emission standards. Nowadays being classified as a grade-A sewage plant has become a goal of many sewage plant managers, and this is why we have seen an increase in the use of water treatment chemicals.

Under the pressure of rising emission standards, the market size of water treatment chemicals is anticipated to rise significantly. According to the forecast of the Prospective Industry Research Institute, the market size of water treatment chemicals will reach 47.6 billion yuan in 2025.

Due to the leakage of the pipeline network, low COD and low C/N in the influent water are very common in China. This causes the denitrifying bacteria to lack nutrients, and slow working efficiency, ultimately exceeding the standard of total nitrogen. To improve the denitrification efficiency, the sewage treatment plant adds an external carbon source.

There is nothing wrong with the use of external carbon sources, but if you rely too much on it, there will often be two consequences:

1#Unnecessary costs

Many WWTPs blindly add external carbon sources without fully utilizing the internal carbon sources. Most of the external carbon sources are used to consume excess aerated oxygen, and a large number of carbon sources are wasted, increasing the operating costs of the WWTPs.

It is estimated that for a 100,000 m³/d wastewater treatment plant it would cost about 3 million RMB (about 450,000 USD) or more annually to purchase the equivalent of 20 mg-HAc/L acetic acids (HAc).

 2#Excessive addition of external carbon sources, which will lead to secondary pollution

In 2018, the sewage treatment plant in Jishan Industrial Park, Pengze County added a high quantity of carbon sources to the sewage system because the total nitrogen value was as high as 60.5mg/L, resulting in the COD effluent exceeding the standard, and was fined 2.4 million yuan.

A large dosage can also cause sludge expansion in sewage treatment plants. Because a high quantity of carbon source enters the aeration tank, it will cause the expansion of non-filamentous bacteria, which is the so-called expansion caused by the impact load. One of the consequences of this expansion is that the secondary sedimentation tank is full of foam and that the COD of the effluent will increase.

Biological phosphorus removal

In the past few years, when it comes to phosphorus removal, many believe that phosphorus removal must rely on chemicals because biological treatment isn’t efficient enough.

Biological phosphorus removal and nitrogen removal require two completely different treatments, which means that the sewage treatment plant needs to have strong operational capabilities to meet the targets.

Chemical phosphorus removal has two major shortcomings:

1#Unnecessary costs

In the last couple of years, to cope with the total phosphorus index, almost all sewage plants are using phosphorus removal agents, and they are generally excessively added. There are all kinds of phosphorus removal agents on the market without a clear list of ingredients. Based on the principle that more is better, most sewage plants add an excessive amount of chemicals, therefore phosphorus removal has become a major expense for WWTPs.

2#The increase of discharged sludge

The data show that chemical phosphorus removal produces 20% to 30% more sludge than biological phosphorus removal.

The accumulated sludge is gradually overwhelming the sewage treatment plant. The problem of a sludge outlet cannot be solved, and it can only be stuffed in the aeration tank. The phosphorus in the returning sludge will be hydrolyzed again, increasing the total phosphorus concentration in the system, and inhibiting the activity of the sludge, thus requiring more carbon source and phosphorus removal agents, and an increase in the dosage of sludge dewatering agents.

Furthermore, the sludge activity decreases and the flocculation becomes poor. With the overflow of the secondary sedimentation tank, the effluent of the sludge either exceeds the standard or enters the advanced treatment unit, which increases the amount of backwashing water.

What should we do if the COD exceeds the standard?

The actual ingredient is sodium chlorate, which is a strong oxidant. The CODcr method is commonly used in the experiment to determine the COD content. The process includes adding potassium dichromate, a strong oxidant, to the water sample. Under the condition of high temperature and strong acid, the more potassium dichromate is consumed, the higher the COD concentration.

Sodium chlorate is more oxidizing than potassium chromate, so COD reacts with sodium chlorate first, and the consumption of potassium dichromate decreases.

In other words, the "COD remover" only works during detection and is a fraudulent method used to evade supervision.

In January last year, the Shenmu Sewage Plant in Shaanxi Province was fined for adding 131 tons of "COD remover". The relevant authorities identified the incident as "evading supervision and illegal discharge of pollutants by falsifying monitoring data".

However, this still hasn’t deter WWTPs from using the product. "COD removers" are still available on Taobao and Alibaba, with overwhelmingly positive reviews and sales. The customer service will however remind you that if you are worried about being caught using it, they can deliver it to you in a blank bag. The fines for using it are pretty steep though. Back in 2018, a chemical factory in Yibin purchased 2 tons of “COD remover” in a blank bag, without a packaging label. The factory was fined more than 40 million yuan.

"COD remover"

The lesson that we can take from this is that the biological treatment shouldn’t be discarded, but should be given a priority. Before adding any chemicals and external carbon sources, WWTPs should first make full use of internal carbon sources; when using chemical phosphorus removal, biological phosphorus removal should be considered first. Furthermore, before using "COD remover", it should be considered whether the COD exceeds the standard due to the use of a large number of chemicals?