Texas Waste Water Treatment Ch#10 Advanced Treatment Processes

Complex organic compounds, metallic salts, and other chemicals invade our waterways with increasing frequency. They enter the drinking water and eventually wastewater. Primary and secondary waste treatment processes may not effectively remove these chemicals. For many years, dilution and purification of the effluent in the receiving stream was considered acceptable, but natural processes can no longer be relied on. Something better than primary and secondary wastewater treatment is often needed. In these cases third-stage (tertiary) wastewater treatment becomes necessary.

Direct filtration of secondary effluent is often used to reduce suspended solids to low levels. It is one of the most efficient processes for upgrading plant performance and can be used for ponds, activated sludge plants, trickling filters, and RBC units. Direct filtration may be necessary whenever a plant is required to consistently maintain TSS values below 15 mg/L. There are several types of effluent filtration devices, but deep-bed, high-rate filters, pressure filters, and shallow-bed traveling bridge filters are commonly found in Texas.

The most important factor affecting filter performance is the quality of the secondary effluent applied to the filter. If the effluent from the biological treatment process is consistently low in solids, filters will perform well and require less frequent backwashing. If the filter receives excessive amounts of poor solids or if the treatment process experiences many upsets, filtration will be difficult. When operating properly, a filtration process should remove about 70%–90% of the Total Suspended Solids(TSS) applied. Factors that affect filter performance are influent quality (such as TSS concentration, floc size, floc strength, and temperature), rate of filtration, characteristics of the filtration media (such as its depth, particle size distribution, porosity, and specific gravity), media type (single, dual, or multimedia. Multimedia filters use several types of filter media, including silica sand, garnet sand, anthracite coal, and granular activated carbon), head loss through the filter, and adequacy and frequency of filter cleaning.

Proper cleaning is essential to filter performance. Ineffective cleaning leads to problems such as filter breakthrough, mud balls, slime buildup, and filter media cracking. Filters are cleaned by backwashing. When the loss of head through a filter is high or when the filter has accumulated too many solids, it must be backwashed. This involves removing the accumulated solids by the rapid upflow of clean wash water through the filter media. In wastewater filtration, backwash with surface wash and periodic shock chlorination is required to maintain proper operation of the filter.

Adsorption occurs when a material clings to the surface of a second material, usually a solid. If the solid holds the adsorbed material and removes it from water, purification occurs. Adsorption is a very effective way to remove organics from water and has been used for many years to purify drinking water and other beverages. Activated carbon is the adsorbent most commonly used for water treatment. Its uses include removing taste and odor compounds from domestic water supplies and removing small concentrations of organics from wastewater effluent. Activated carbon is an effective adsorbent because of its carbon base, large surface area, and pore size distribution. It retains adsorbed organics well and can be regenerated to restore its effectiveness. Activated charcoal/carbon has a great deal in common with charcoal, but there are some key differences between the two as well. While charcoal is traditionally made from wood and an imcomplete combustion reaction, activated carbon may be made from wood, peat, nutshells, coconut husks, lignite, coal, coir, or petroleum pitch and undergoes an acid bath that thins the material further creating more overall surface area to act on the material passing through it.

The use of granular activated carbon (GAC) for organic removal is common. Organic compounds removed by GAC adsorption include carbohydrates, proteins, and fats. It is also effective in removing organic colors from tannins and lignins (decaying vegetation). Through adsorption, it is possible to remove 75%–80% of organics from secondary clarifier effluent.

Biological growth will quickly plug the pores in the GAC bed reducing its adsorption capabilities.

Chemical oxidisation involves adding or generating oxidants in the wastewater. The goal being to find one that can be efficiently and economically obtained and employed. A few currently used oxidants include ozone (O3), hydrogen peroxide (H2O2), natriumhypochlorite or bleaching liquor (NaOCl), chlorine dioxide (ClO2), chlorine gas (Cl2), peroxy acetic acid (C2H4O3) and pure oxygen (O2).

Many overlook ozone as an oxidant because of its relative ineffectiveness in conventional waste treatment. However, a third stage of sewage treatment will start with an effluent that is relatively low in organic matter and other impurities. In such circumstances, the high oxidizing power of ozone may operate particularly well. Furthermore, new methods of producing ozone will no doubt considerably reduce the cost of ozonation.

Laboratory-scale studies have shown that the changes occurring in the settlement of the floc are greatly influenced by biological activity. If the process is dependent on biological activity, then temperature and the presence or absence of toxic material, both of which affect biological activity, become important parameters.

It has been recommended that chemical precipitation to remove finely divided suspended solids and colloids not be practiced indiscriminately. Studies show that with long detention times, small dosages of lime or acid may actually hinder the degree of purification. Some commonly used coagulants include aluminum sulfate (alum), sodium aluminate, and ferric sulfate. Coal ash activated with sulfuric acid has also been used with some degree of success. The main problem with using coagulation in further treating wastewater effluent is the lack of knowledge concerning its effectiveness in the removal of soluble impurities. Additional information on Chemical Precipitation can be found in the EPA Fact Sheets on the Reference CD included when you sign up for the course with TEEX.

"Osmosis occurs when solutions of two different concentrations are separated by a semipermeable membrane, such as cellophane." - from the TEEX Waste water treatment textbook for additional information i recommend https://en.wikipedia.org/wiki/Osmosis and https://en.wikipedia.org/wiki/Reverse_osmosis

The biological process of denitrification involves the conversion of nitrate nitrogen ions (NO3-) to gaseous nitrogen (N). The gaseous product is primarily nitrogen gas, but some nitrous oxide or nitric oxide may also result during denitrification (Figure 10.7). Biological denitrification is accomplished under anaerobic conditions by heterotrophic bacteria that use nitrate during the fermentation of organic carbon materials.

This excerpt applies to the nitrate removal process and specifically the role facultative bacteria play in it. Electrons pass from the carbon source (the electron donor) to nitrate or nitrite (the electron acceptor) to promote the conversion to nitrogen gas. This involves the nitrifiers “electron transport system” and releases energy from the carbon source for use in organism growth. This electron transport system is similar to that used for respiration by organisms oxidizing organic matter aerobically, except for one enzyme. Because of this close relationship, many facultative bacteria can shift between using oxygen or nitrate (or nitrite) rapidly and without difficulty.

During biological treatment, significant changes take place. As organic materials are decomposed, their phosphorus content is converted to orthophosphate. Inorganic phosphates are utilized in forming biological floc. The polyphosphates are for the most part converted to orthophosphate. The result is that, in a well-treated secondary effluent, a large fraction of the phosphorus is present as orthophosphate, which is the easiest form to precipitate.

Current chemical phosphorus removal design is based on equilibrium precipitation theory (WEF 1997, 1998; USEPA 1987a, 1987b). The chemical precipitate of ortho-phosphorus is carried by treatment with a trivalent metal cation, typically ferric ion (Fe3+) or aluminum (Al3+). Ferric ion is typically supplied in the form of ferric chloride (FeCl3). Aluminum is supplied as alum (aluminum sulfate). When a source such as waste pickle liquor is available, the ferrous ion (Fe2+) can be used as the metal cation. The precipitation reaction depends on the various phosphate species (e.g., H2PO4 1-, HPO4 2-) being converted to PO4 3-, with the consumption of alkalinity (or OH-, hydroxide ion). This means that sufficient alkalinity must be present for the chemical precipitation reaction to be completed. The materials found practical for phosphorus precipitation include the ionic forms of aluminum, iron, and calcium. The calcium is added as lime.