What is the proportion of make-up water in the circulating water of general manufacturing enterprises?

I work in the papermaking industry, and I’ll share my experience using papermaking as an example. The general principle is that the proportion of COD carried away by the water system corresponds to the approximate proportion of recycled water. For example, if the proportion of COD carried away by the paper in our paper production process is 85%, then our goal for water recycling is to achieve 85% water reuse. If this percentage is lower, retention agents are used to increase the retention rate of the pulp during its processing, thereby reducing the amount of COD that is carried away. Another principle is that the COD of the external wastewater cannot be higher than that of the paper machine white water; otherwise, there will be available fresh water. Therefore, you first need to determine the water balance of your enterprise, identify the proportion of COD that is removed, and then find ways to increase this removal rate without affecting production, raise the proportion of recycled water, reduce the amount of water that needs to be added, and thus lower consumption. The above are my personal experiences; I’m not sure if it’s the same in chemical companies.

As I understand it, the circulating water used for cooling is mainly applied in places that require cooling, such as heat exchangers. The proportion of make-up water depends primarily on local weather conditions and design requirements; it simply needs to be adjusted according to the concentration ratio. Generally, the proportion of make-up water varies depending on the season. As for the more detailed aspects, I still haven’t figured them out yet, haha!

The amount of water to be replenished is determined by the losses in the system, including evaporation from the cooling tower, leaks from users, and the amount of water required for system replacement. Different factories have different situations, so there is no universal rule; the evaporation and leakage amounts can be determined based on the drop in the liquid level of the cooling tower, while the amount of water needed for replacement is determined according to the requirements regarding water quality.

Our company has recently undertaken a project to produce dimethyl ether through the gas-phase dehydration of methanol; a contract has already been signed. In the plan sent by the design firm, the volume of circulating water is 4,500 tons, while the water lost due to evaporation and other factors is 50 tons. The amount of water added to the circulating water system, as per their design, accounts for 1% of the total circulating water volume. However, water is generated as a by-product of the dimethyl ether production process, so about 30 tons of water needs to be added as a supplement.

Replenishing water in a circulating system is very simple; high precision isn’t necessarily required. The circulating water system should have cooling fans and a tank; a level switch can be installed in the tank to control a pneumatic valve via interlocking. Set a high level as the low level and another level as the high level; control it using a level switch. When there is less water, it will automatically be replenished, and once the high level is reached, the valve will close automatically to stop further addition. This design is very simple and easy to implement. The pipeline for supplying water can be designed to require a higher flow rate, so there is no need to worry about the water level in the sink running low or overflowing. One advantage of such a design is that it operates regardless of the season, and it is not affected by leaks or other losses or an increase in water volume; it is fully automated. This post was last edited by ntsdn on 2007-11-11 08:39]

Cooling water serves primarily to dissipate heat and to supplement the water used for wastewater treatment; its role is mainly to keep the water quality such that the total hardness remains below 4500 PPM. The rate of water supplementation mentioned by the original poster generally depends on the quality of the water being used for supplementation (i.e., the total hardness of the raw water), and the so-called concentration factor is calculated as 4500/total hardness of the raw water

The water losses in circulating water include evaporation + wastewater discharge + leakage + splashing and dripping, among which evaporation and wastewater discharge are the two main sources of loss. Evaporation accounts for about 1% of the amount of circulating water, and it is primarily influenced by the system’s thermal load and the season； The wastewater discharge rate ranges from 0.1 to 0.5%, and it is mainly related to the concentration factor of the circulating water. Other water losses are minimal, so the amount of water required to replenish the circulating water is 1.1–1.5% of the volume of circulating water.

It is generally around 2%. A simple estimation can be made as follows: for a temperature drop of 10 degrees, to lower the temperature of 1 ton of water by 10 degrees using a cooling tower, the heat required is Q=1000*10=10,000 kcal. The amount of water that needs to be evaporated to obtain this amount of heat is W=10,000/539=18.55 kg. Since 18.55/1018.55=1.8%, and considering factors such as concentrated wastewater and water vapor loss, the amount of water added back usually remains above 2%.

The concept of make-up water is related to different industries; I work in an ethanol production company, and make-up water includes losses from recycled water, consumption in the manufacturing process, other types of consumption, as well as water used for domestic purposes. The replenishment of circulating water is mainly related to the concentration ratio ； The process water consumption is used in production and participates in the reactions; this amount must be replenished, and it also depends on the technical level in the same industry ； The water consumption for domestic use is generally low ； Other consumptions include leaks, flushing water, etc ； In the better ethanol production industries, the fresh water consumption is generally around 20 tons per ton of ethanol.

I work at a refinery in the Northeast; we take 2.5% of the designed capacity into account.