Wastewater and Sludge

Wastewater and Sludge

Wastewater treatment is the process of removing contaminants from wastewater. It includes physical, chemical and biological processes to eliminate pollutants. Once treated, water can then be released back into the environment. Sewage is created by residences, institutions, hospitals, commercial and industrial establishments. It can be treated close to where it is created or collected and transported via a network of pipes and pumping stations to a municipal treatment plant. As far as industrial sources of wastewater are concerned, they often require specialized treatment processes, which are frequently present on site.

Neutralac® is the global Lhoist brand for products and services used in the treatment of waste and sludge. Its role is to optimize the further settling of various materials and reduce and neutralize bad odors.

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Wastewater neutralization

Wastewater must be neutralized to adjust its pH value. Only by doing this, it can meet the requirements of different processing units within a wastewater treatment system.

Neutralization may be used to treat acid wastewaters containing metals. Adding an alkaline reagent increases the pH of the acid waste. This forms a precipitate that collects unwanted metals. The result is an incoming solution of which the pH has been adjusted to the optimum range for precipitating metals as hydroxides.

Neutralac® SLS45 represents an excellent example of the Lhoist’s R&D approach to the development of innovative solutions. As a fast-acting, ready-to-use suspension of 45% solids, it is delivered at a viscosity below 300 cPs.

Featuring high reactivity (KIWA T90 < 5 sec), it is ideal for rapid acid neutralization and is also proficient at maintaining a steady pH.

Neutralac® SLS45 is more advanced than other liquid lime currently available on the market and is capable of performance comparable to caustic soda. In many cases, the use of Neutralac® SLS45 improves the precipitation of heavy metals, flocculant stability and filter cake moisture content and porosity.

To learn more, visit: www.sls45.com

 

Wastewater precipitation

Reduction of heavy metals and other ions

Metals do not dissolve in the environment. They can be very toxic to humans and animals. Municipalities and industries must therefore remove them from wastewater.

Chemical precipitation is the most common technology used to eliminate (ionic) metals from solutions, such as process wastewaters that contain toxic metals. The ionic metals are converted to an insoluble form (particle) by the chemical reaction between the soluble metal compounds and the precipitating reagent. Particles formed by this reaction are removed from the solution by settling and/or filtration.

Several factors influence the effectiveness of a chemical precipitation process, including the type and concentration of ionic metals present in solution, the precipitant used, the reaction conditions (especially the pH of the solution) and the presence of other constituents that may inhibit the precipitation reaction.

Hydroxide precipitation is the most widely used chemical precipitation process. During this process, metal hydroxides are formed by using calcium hydroxide as the precipitant. Each dissolved metal has a distinct pH value at which the optimum hydroxide precipitation occurs - from 7.5 for chromium to 11.0 for cadmium.

When considering the cost of precipitating agents and alkali requirements, lime is the most cost-effective option to remove metals from wastewater.

Sludge treatment

The sludge accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. Which sludge treatment methodology to use depends upon the amount of sludge generated and other site-specific conditions.

Lime simplifies the dewatering of organic and mineral sludge by increasing its dry matter content and improving its structure (simpler handling and storage). It also stabilizes and sanitizes sludge in water treatment plants to avoid odors, thereby enhancing its agricultural value.

You can use lime to stabilize industrial residual sludge, for sludge dredging and to treat urban biosolids before agricultural re-use or incineration. Helping coagulation, lime makes sludge easier to handle, prevents fermentation, limits odors, eliminates pathogens and holds metallic ions in an insoluble form. It also enriches the sludge with calcium and magnesium (when dolomitic lime is used), both of which are particularly beneficial for agricultural reuse.

The design of a sludge management and minimization strategy must aim for reduction at every step of the sludge generation chain:

  • throughout the production process, to reduce waste and then to reduce sludge production
  • in the wastewater treatment plant, by using new adapted technologies to reduce the amount of sludge produced per amount of Biological Oxygen Demand (BOD) treated
  • during disposal, to select the most stable and most economical route.

Adding Neutralac® to sludge offers many benefits:

Polluted soil treatment

Soil contamination is caused by the presence of man-made chemicals or other modifications to the soil. It typically arises from the rupture of underground storage tanks, the application of pesticides, percolation of contaminated surface water to subsurface strata, oil and fuel dumping, leaching of waste from landfills or direct discharge of industrial wastes to the soil. The most common chemicals found are petroleum hydrocarbons, solvents, pesticides, lead and other heavy metals. The chances of soil contamination are related to the degree of industrialization of an area and the intensity of chemical usage in the vicinity.

Concerns about soil contamination primarily come from potential health risks, both from direct contact with and secondary contamination of water supplies.

Cleanup of environmental pollution involves a variety of techniques, ranging from simple biological processes to advanced engineering technologies. Elimination processes may be based on the specific contaminant (or contaminant class) and site.

Metallurgical residues

With increasingly strict environmental legislation, raw material shortages and high metal prices, the implementation of dedicated processes for treating metallurgical residues and recovering their valuable content is becoming more and more relevant. Lhoist engineers, in association with our partners, work on innovative processes to destroy hydrocarbons contained in various forms of sludge as well as the recovery of certain metals, ready for re-use in the plant.

For example, oily sludge and scale form part of metallurgical residues. Today, it is possible to recover their iron content in an efficient way without impeding the main iron-steel flow sheet.