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 located on site.

Lhoist’s SLS45® product is 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.

Wastewater neutralization

Acidic wastewater is neutralized using a strong base in order to be further process within a wastewater treatment system.  This requires the use of an alkaline reagent or base.  Lime, as calcium hydroxide, is commonly used as an alkaline neutralizing reagent.  In addition to neutralization, where acidic wastewater contains metals, adding an alkaline reagent results in the precipitation of the unwanted metal as hydroxides.  SLS45® is an excellent example of an  innovative product developed by Lhoist for wastewater neutralization.  It is a fast-acting, ready-to-use suspension or slurry of 45% calcium hydroxide solids, delivered at a viscosity less than 500 cPs.  It is ideal for rapid acid neutralization (KIWA T90 < 5 sec), resulting in a steady neutral pH.

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

To learn more, visit: www.sls45.com


Wastewater precipitation

Reduction of heavy metals and other ions

Municipalities and industries treating wastewater remove heavy metals through chemical precipitation.  Industrial water containing heavy metals as ionic metals are most commonly acidic.  Using an alkaline reagent (base) as a precipitating reagent the acid is neutralized and the ionic heavy metals are converted to an insoluble form (particle).  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 precipitating agent. Each type of dissolved heavy metal has a distinct pH value at which the optimum hydroxide precipitation occurs – ranging 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 heavy 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 SLS45® to sludge offers many benefits:

Polluted soil treatment

Soil contaminated with pollution can arise from the rupture of underground chemical 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.  Soil contamination can be  related to the degree of industrialization of an area and the intensity of chemical usage in the vicinity.

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 increasingly important. Lhoist engineers, in association with our partners, work on innovative processes to destroy hydrocarbons contained in various forms of sludge as well as on 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.

Acid rock drainage and acid lake treatment

Acid rock drainage (or acid mine drainage) is generally caused by the bio-catalyzed oxidation of pyrite in disturbed conditions. This occurs where pyrite-containing rock is exposed to a combination of water and oxygen (from air). The oxidation of pyrite generates sulfuric acid. This can dissolve a range of toxic metals that could pollute groundwater, rivers, lakes and oceans. Lime is a key reagent used to remediate such scenarios. Lhoist has 20 years of experience in neutralizing acidic lakes, applying its products with specially developed boats. www.neutralac.com