Effective Management of Wastewater Treatment Plants

Posted on Dec 6, 2024 in Chemistry

Treatment Plant Management

Wastewater treatment plants are two types of property: private and public.

The former seek to be the latest generation, high efficiency, so that the minimum operational costs, with highly qualified and understaffed personnel, highly automated systems with fast response to any eventuality, and ease of replacement and modernization.

The public type is subject to the availability of funds and therefore is not generally high in technology, and its efficiency is not high. Work facilities are preferred by those low-tech and are operated by unskilled personnel, but with specific training. The availability of repair and modernization is slow, subject to bureaucratic procedures and tenders, as well as budgetary constraints.

Plant Management

The costs to operate a treatment plant can be separated into two concepts:
Start-up costs (infrastructure) and operational costs.

First, consider the costs of feasibility studies, basic engineering, project execution, and the construction of the plant. This is generally regarded as a global cost, called the cost of Infrastructure (CI).

The latter corresponds to the operation of the plant and considers the following:

• Operating Cost (OC): Reagents, energy, fuel, wages, and salaries of personnel directly involved in the operation of the plant.
• Cost of Administration (CA): Stationery, telephones, vehicles, fuel, wages, and salaries of employees not directly involved in the operation of the plant.
• Maintenance Costs (MC): Corrective and preventive maintenance of equipment, systems, and power lines.

The unit cost or cost per cubic meter is determined according to the formula:

CU = (CO + CA + CM) / (KV) + CI / (K Vu V)

where:

• K: The collection efficiency ratio, i.e., the percentage of users who pay or can afford the service, minus those who are late or who do not pay.
• V: The annual volume treated by the plant.
• Vu: The lifetime of the plant.

The costs are generally less than $1/m3 when the plant is operated efficiently and effectively.

See the following example:

CO = $3,000,000 per year
MC = $1,500,000 per year
CA = $2,000,000 per year
CI = $125,000,000
Operating expenditure: 800 lps = 0.8 * 86400 * 365 = 25,228,800 m3

Life Expectancy: 20 years
K = 0.93

CU = (3,000,000 + 1,500,000 + 2,000,000) / (25,228,800 * 0.93) + 125,000,000 / (20 * 25,228,800 * 0.93)

CU = $0.54/m3
Advanced Wastewater Treatment

Many of the substances found in wastewater are affected by little or no processes or operations in conventional treatments. These substances range from relatively simple inorganic ions such as calcium, potassium, nitrate, sulfate, and phosphate to a growing number of complex synthetic organic compounds.

Even the effect of these substances on the environment is not well understood; the demands for more rigorous treatments will be referred to as the tolerable concentration of many of these substances in the effluent of the plants.

The following table will show some typical chemical components that can be found in wastewater and their effects:

Tertiary or advanced treatment is of great interest today due to the need for better water quality. For these reasons, some successful processes used at present or that seem most promising or innovative are presented below:

Distillation

Distillation is a unit operation in which the liquid solution components are separated by vaporization and condensation of liquid.

Foam Fractionation

Foam fractionation means separating colloidal and suspended matter by floating and dissolved organic matter by adsorption. When air bubbles in water, it produces residual foam, or this is induced by chemicals. Almost all organic compounds have surface activity; they tend to focus on gas-liquid interfaces and are removed along with the foam.

Freezing

Freezing is an operation similar to distillation separation. The water is sprayed in a vacuum-operated chamber. Part of the residual water evaporates, and the cooling effect of ice crystals is produced without contaminants in the liquid that remains. Ice is then extracted and fused by heat of condensation of the vapor from the evaporation stage. In this procedure, butane and other refrigerants have been used.

Ion Exchange

Ion exchange is a process in which ions remain bound to functional groups on the solid surface by electrostatic forces exchanged by different species in solution. Since demineralization can be accomplished by ion exchange processes, current treatments can use part of the wastewater effluent demineralized and combined with part of the effluent after it has been diverted to treatment to produce an effluent quality specified.

Electrochemical Treatment

In this process, wastewater is mixed with seawater and passed through a single cell containing carbon electrodes. Because of the relative densities of seawater and the mixture of seawater and wastewater, the first accumulates on the surface of the anode at the bottom of the cell, while the latter accumulates on the cathode surface near the top of the cell. The current raises the pH in the cathode, thereby precipitating phosphorus and ammonia. Hydrogen bubbles generated at the cathode raise the sludge to the surface, where it is washed and removed by conventional methods. The chlorine evolved at the anode of the cell disinfects the effluent, and the excess mixed seawater sewage is then discharged into the sea.

Tertiary Treatment

If your water is to receive the discharge, it requires a degree of treatment greater than that available in the secondary process, or where the effluent will be reused, you need advanced treatment of wastewater. Often the term is used synonymously with tertiary treatment, but they are not exactly the same. Tertiary treatment, or the third phase, is usually used to remove phosphorus, while advanced treatment may include additional steps to improve effluent quality by removing recalcitrant pollutants. There are processes that can remove over 99% of suspended solids and reduce the BOD5 in a similar measure. Dissolved solids are reduced by processes such as reverse osmosis and electrodialysis. The elimination of ammonia, denitrification, and precipitation of phosphate can reduce the nutrient content. If you plan to reuse wastewater, disinfection by ozone treatment is considered the most reliable, except for extreme chlorination. It is likely that future widespread use of these and other methods of waste treatment will be made in light of efforts being made to conserve water through reuse.

Liquid Spill

The final discharge of treated water is done in several ways. The most common is the direct discharge to a river or lake receiver. In parts of the world facing a growing shortage of water for both domestic and industrial use, the authorities are beginning to resort to the reuse of treated water to fill aquifers, irrigate non-edible crops, industrial processing, recreation, and other uses. In a project of this type, the Potable Reuse Demonstration Plant in Denver, Colorado, the treatment process includes primary and secondary conventional treatments, followed by a cleaning lime to remove suspended organic compounds. During this process, an alkaline environment (high pH) is created to enhance the process. In the next step, carbonation is used to return to a neutral pH. Then the water is filtered through multiple layers of sand and charcoal, and ammonia is removed by ionization. Pesticides and other organic compounds are absorbed even in suspension by a granular activated carbon filter. Viruses and bacteria are removed by ozonation. At this stage, the water should be free of all contaminants, but for safety, the second phase uses carbon absorption and reverse osmosis, and finally, chlorine dioxide is added to obtain high-quality water.

Septic Tank

A process of treating wastewater that is often used for household waste is the septic tank: a pit of cement, brick, or metal blocks in which the solids settle and floating material accumulates. The partly clarified liquid flows through a submerged outlet to underground trenches filled with rocks through which it flows and seeps into the ground where it is oxidized aerobically. The solids and floating material deposited may be retained for six months to several years, during which they decompose anaerobically.

Tertiary Treatment

The reuse of wastewater by tertiary treatment is a good alternative to save water and reduce consumption considerably. Key to this is bringing the water to the plant output parameters suitable for use for other purposes, such as watering the garden, because fundamentally, the treated water will present a bacteriological content and therefore needs to be disinfected. Hidritec has basically three methods of disinfection of water that can be complemented with proper filtration, studying each case separately depending on the type of wastewater and specific needs.

Tertiary Treatment by Chlorination System

This method tries to keep purified water in a reservoir with a final distribution of free chlorine content that is appropriate to prevent the proliferation of microorganisms in order to make it suitable for reuse. There are several forms of chlorination of the tank, which can go through automatic measurement and control of the dosage of chlorine in the tank by a probe of free chlorine or chlorine dosage redox or proportional to the flow of purified water by installing a counter-pulse emitter. The chlorination of wastewater is the easiest and most economical system for tertiary treatment of water reuse for irrigation of gardens and plants. A noteworthy disadvantage is that it requires the use and manipulation of a chemical like sodium hypochlorite. In addition, certain ornamental plants, vegetables, or fruit crops may be susceptible to being damaged from certain levels of free chlorine. Also noteworthy is that this system always implies the exclusive use of a tank for chlorination, as it is always a need for adequate contact time of chlorinated water to ensure disinfection.

Tertiary Treatment Using Ultraviolet Radiation

In this case, the disinfection is performed using UV equipment that provides immediate disinfection and is more effective than chlorination. Another added advantage is that it requires no contact deposits since disinfection is performed instantaneously by passing water through ultraviolet treatment equipment, which encourages this type of tertiary treatment when there is not enough space for treatment with chlorine or ozone. To ensure the proper functioning of ultraviolet equipment, a good filtration system should be in place to remove turbidity and ensure adequate transmittance of ultraviolet radiation on the flow of water to be treated.

Tertiary Treatment by Ozonation

Ozone is a powerful oxidant and disinfectant with a sterilization rate higher than conventional chlorine treatment, which increases its effectiveness. This allows ozone treatments with very small contact tanks, as only about three minutes of contact time is needed to ensure disinfection. In addition, for the treatment of wastewater for reuse in irrigation and agriculture, ozone provides greater oxygenation to the roots of the plant while conveying its disinfectant character. The results are crops with faster growth, higher productivity, and avoidance of pests and diseases.

Tertiary Treatment of Sewage

The operations used in the tertiary treatment of contaminated water include: microfiltration, coagulation and precipitation, activated carbon adsorption, ion exchange, reverse osmosis, electrodialysis, nutrient removal, chlorination, and ozonation. Any treatment of sewage that is performed after the secondary stage is called tertiary treatment, and in this, it seeks to eliminate organic pollutants, nutrients like phosphate and nitrate ions, or excess salts. In the tertiary treatment of sewage waste, the goal is to be as pure as possible before being dumped into the environment. In the treatment of wastewater to remove nutrients, the processes include precipitation, sedimentation, and filtration. Currently, tertiary treatment for domestic sewage is very low.

Chlorination Process

The method of chlorination is the most used, but as chlorine reacts with organic matter in wastewater and surface water, it produces small amounts of carcinogenic hydrocarbons. Other disinfectants such as ozone, hydrogen peroxide, and UV light are beginning to be used in some places, but they are more expensive than chlorination. While elemental chlorine or atomic form can be used for water disinfection, some of the compounds of chlorine, such as hypochlorous acid, sodium hypochlorite, calcium hypochlorite, and chlorine peroxide, are more commonly used. Some of the chemical reactions between compounds of chlorine and water are represented in the following chemical equations:

Hydrolysis of chlorine: Cl2 + 2 H2O → HCl + H3O+

Dissociation of hypochlorous acid: HClO + H2O → H3O+ + ClO-

Acidification of sodium hypochlorite: NaOCl + H+ → HClO + Na+

Chlorine with ammonia can form chloramines, which also have disinfecting action. Chlorine peroxide is also able to oxidize phenols. Chlorine has a toxic action on microorganisms and acts as an oxidant on the non-degraded organic matter and some minerals. Chlorine destroys but does not sterilize pathogenic microorganisms because it does not affect saprophytes.

Biological treatment can be summarized in the following diagram: