CE 40340 Wastewater Design Spring 2009 Wastewater Solids X a = concentration of active biomass Mx L3 Volatile, partially biodegradable X i = concentration of inert (non-active) biomass Mx L3 Volatile, refractory to degradation X v = Total volatile solids = X a X i Mx L3 mg vss a / L Volatile solids expressed as mg vss L mg vss i / L mg vss / L Wastewater solids also exert COD and BOD, and contain N and P. COD COD is exerted by both active and inert VSS. Given the approximate formula for biomass: C5H7O2N, with a MW = 113 mg/mmol and considering non-nitrifying conditions, there are 20 electron equivalents per mol of biomass: 1mgVSS 20e meq mgCOD mgCOD * *8 1.42 mgVSS mmolbiomass mgVSS e meq 113 mmolbiomass Therefore, there is 1.42 mgCOD/mgVSS, without considering nitrification If oxidation of the reduced N in biomass to NO3- is considered, then there are 28 emeq/mol biomass, and the conversion becomes 1.98 mgCOD/mgVSS considering nitrification 1 CE 40340 Wastewater Design Spring 2009 BOD Only the degradable fraction (fd) of active biomass (Xa) exerts BOD. Inert biomass (Xi) does not exert BOD. the ultimate BOD (BODu) of active biomass expressed as VSS is 1.42fd mgBODu/mgVSSa without nitrification, and 1.98fd mgBODu/mgVSSa with nitrification A typical value of fd is 0.8. Nutrients Nitrogen. Assuming VSS is mainly cells, then VSS is C5H7O2N. The MW of C5H7O2N is 113 mg/mmol, and considering 14 mgN/N, then there is 14/113 = 0.124 mgN/mgVSS Phosphorus. Phosphorus is typically 20% by mass of N, therefore 0.025 mgP/mgVSS 2 CE 40340 Wastewater Design Spring 2009 Relationship between VSS and TSS VSS is measured by the mass that is volatilized at 500 oC, which corresponds to the organic fraction (C5H7O2N). However, cells also contain around 10 % inorganic matter that is non–volatile. To convert microbial VSS to TSS : cell VSS 0.1 TSS VSS 0.1 VSS1 0.9 0.9 Organic, volatile solid = VSS Inorganic, non-volatile solid VSS 0 .1 = 0.9 In wastewater, Mixed Liquor Volatile Suspended Solids (MLVSS) is a measure of X v . Mixed Liquor Suspended Solids (MLSS) is a measure of total solids, i.e., organic and inorganic. This includes - X v (MLVSS) 0.1 MLSS - Inorganic fraction of biomass = MLVSS 0.9 - Inorganic suspended solids from influent SSinorg. In an activated sludge system with solids recycle, the influent inorganic suspended solids are concentrated by solids concentration factor 0.1 MLSS MLVSS 1 SSinorg. x 0.9 3 x