E-Andrew Sindt Creative Component S11.pdf

Chapter 2: Literature Review2.1 Nutrient Removal in Wastewater Treatment Plants2.1.1 Nitrogen RemovalInfluent nitrogen concentrations, predominantly in the forms of ammonia (NH 3 ), ammonium (NH 4 + ), andorganic nitrogen are removed via two mechanisms. The first mechanism is the natural physiologicalnitrogen requirement for cell metabolism and growth. The second is the combination of nitrification,conversion of ammonium to nitrite and nitrate, and denitrification, the conversion of nitrate to nitrogengas. Nitrification and denitrification will be discussed further in detail in the section Biological NutrientRemoval.2.1.2 Phosphorus RemovalPhosphorus removal from wastewater streams relies on converting soluble phosphorus species toparticulate form, then removing the particulate through settling, conventional filters, and/or membraneprocesses. Orthophosphate, soluble in nature, is the most dominant phosphorus species in wastewaterand is used for biological metabolism. Additionally, other forms of phosphorus are converted toorthophosphates which are also available to support biological growth (Neethling, 2008). Thus, solublephosphorus is taken up by the biomass and held in particulate form amounting to 1-2% of the totalsuspended solids mass in the mixed liquor (Lesjean et al., 2003). This is also reflected by thestoichiometric formula for biomass given as C 5 H 7 O 2 NP 0.1 (Rittman and McCarty, 2001).Phosphorus uptake into biomass can be further enhanced with appropriate conditions which allow forenhanced biological phosphorus removal (EBPR) or luxury phosphorus uptake. EBPR is defined asphosphorus uptake by bacteria that exceeds the 2.3% phosphorus by weight typical of conventionalactivated sludge type biomass (Randall et al., 1997). EBPR depends on the proliferation of phosphorusaccumulating organisms (PAOs) to increase the phosphorus concentration up to about 7% total solids.The required environment for PAOs to outcompete other microorganisms consists of alternating theoperating conditions from anaerobic to aerobic.Phosphorus can also be removed by physicochemical fixation of phosphate through precipitation andadsorption. This can be achieved naturally (with appropriate pH and presence of cations) or artificiallythrough dosing chemical coagulants such as iron or alum salts. Although chemical phosphorus removalcurrently holds an advantage over EBPR in terms of better control, many drawbacks do exist. Thesedrawbacks include increases in sludge production (up to 25%), chemical costs, and effluent salinity aswell as potential adverse conditions for biological nitrification (low alkalinity and pH). Due to thesenegative aspects associated with chemical phosphorus removal, EBPR is considered a preferredtreatment technology for phosphorus removal.13