Coal Runoff Waste Water Treatment Process

Coal Runoff Waste Water Treatment (WWT) Process


The waste water source of Coal Runoff WWT is from Coal yard sedimentation pond. The process block diagram is as follow :

figure 1. simplified coal runoff block diagram 

There are 6 basic process in Coal Runoff Waste Water Treatment, that are:
    1. Coagulation
    2. Floculation
    3. Clarification or Sedimentation
    4. Filtration
    5. Neutralization
    6. Sludge thickening

Coagulation is destabilization process of suspended solid colloid material with coagulant agent to produce micro floc using rapid-mix.

Floculation is floc formation process which basically uses agglomeration grouping between particles and coagulants with gentle mixing to combine several particles into large floc.

Sedimentation is the process by which suspended solid particle are removed from the water by means of gravity or separation. Sedimentation involves one or more basins, called Clarifier. In properly designed clarifier, the velocity of the water is reduced so that gravity is the predominant  force acting on the water/solid suspension. The key factor in this process is speed. The rate at which a floc particle drops out of the water has to be faster than the rate at which the water flows from the tank's inlet or slow mix end to its outlet or filtration end. The difference in specific gravity between the water and the particles causes the particles to settle to the bottom of the basin. Some plants have added baffles or weirs in the sedimentation basins to limit short-circulating through the basin, promoting better settling.

To maximize the sedimentation rate, usually used Lamella Clarifier. Inclined tubes and plates can be used in sedimentation basins to allow greater loading rate of sedimentation. This technology relies on the theory of reduced-depth sedimentation : particles need only settle to the surface of the tube or plate below for removal from the process flow. Generally, a space of two inches is provided between tube walls or plates to maximize settling efficiency. The typical angle of inclination is about 60 degrees, so that settled solid slide down to the bottom of the basin.

figure 2. lamella clarifier

Filtration is a process used to separate solids from liquids or gases using a filter medium that allows the fluid to pass through but not the solid. The term "filtration" applies whether the filter is mechanical, biological, or physical. The fluid that passes through the filter is called the filtrate. The filter medium may be a surface filter, which is a solid that traps solid particles, or a depth filter, which is a bed of material that traps the solid. Oversize particles may form a filter cake on top of the filter and may also block the filter lattice, preventing the fluid phase from crossing the filter, known as blinding. The size of the largest particles that can successfully pass through a filter is called the effective pore size of that filter. The separation of solid and fluid is imperfect; solids will be contaminated with some fluid and filtrate will contain fine particles (depending on the pore size, filter thickness and biological activity).

Neutralization is a chemical reaction in which acid and base react to form salt and water. Hydrogen (H+) ions and hydroxide (OH- ions) reacts with each other to form water. The strong acid and strong base neutralization have the pH value of 7. Before discharging the waste water, adjast pH is very important. In Indonesia, keep pH around 6.5 - 8.5.

figure 3. pH scale

The Side process is Heavy Metal Precipitation. This happen at the Coagulation than which injected Precipitation Agent. The principle technology to remove metals pollutants from wastewater is by chemical precipitation. Chemical precipitation includes two secondary removal mechanisms, coprecipitation and adsorption . Precipitation processes are characterized by the solubility of the metal to be removed. They are generally designed to precipitate trace metals to their solubility limits and obtain additional removal by coprecipitation and adsorption during the precipitation reaction.        There are many different treatment variables that affect these processes. They include the optimum pH , the type of chemical treatments used, and the number of treatment stages, as well as the temperature and volume of wastewater, and the chemical specifications of the pollutants to be removed.                                                                                                                                                    In theory, the precipitation process has two steps, nucleation followed by particle growth. Nucleation is represented by the appearance of very small particle seeds which are generally composed of 10–100 molecules. Particle growth involves the addition of more atoms or molecules into this particle structure. The rate and extent of this process is dependent upon the temperature and chemical characteristics of the wastewater, such as the concentration of metal initially present and other ionic species present, which can compete with or form soluble complexes with the target metal species.

Heavy metals are present in many industrial wastewaters. Examples of such metals are cadmium , copper , lead , mercury , nickel , and zinc. In general, these metals can be complexed to insoluble species by  adding sulfide, hydroxide, and carbonate ions to a solution. For example, the precipitation of copper (Cu) hydroxide is accomplished by adjusting the pH of the water to above 8, using precipitant chemicals such as lime (Ca(OH)2) or sodium hydroxide (NaOH). Precipitation of metallic carbonate and sulfide species can be accomplished by the addition of calcium carbonate or sodium sulfide. The removal of coprecipitive metals during precipitation of the soluble metals is aided by the presence of solid ferric oxide, which acts as an adsorbent during the precipitation reaction. For example, hydroxide precipitation of ferric chloride can be used as the source of ferric oxide for coprecipitation and adsorption reactions. Precipitation, coprecipitation, and adsorption reactions generate suspended solids which must be separated from the wastewater. Flocculation and clarification are again employed to assist in solids separation.

figure 4. solubility of various metal vs pH

This Process is in asmuch as the soluble metal is react with a reagent (Sulfide) to form an insoluble metal sulfide complex. The reaction is as below :

Sulfide precipitation is often a more effective alternative to hydroxide since the metal sulfide (MS) generally have lower solubility and much higher binding contants. Thus, the sulfide can displace chelants better than hydroxides and force the reaction toward the insoluble precipitate. Very low levels of metals can be achieved with sulfide precipitation. Copper, cadmium,zink, and mercury can be effectively precipitated with short contact time.

Sulfide precipitation uses either a soluble sulfide or an insoluble sulfide as the source ofbthe sulfide reagent. In soluble sulfide precipitation such as Hydrogen Sulfide ( H2S), Sodium Sulfide ( Na2S), or Sodium Hydrogen Sulfide (NaHS) serve as the source of sulfide. In insoluble sulfide such as Ferrous Sulfide (FeS).