Optimal nutrient ratios for wastewater treatment - HACH LANGE

6 NUTRIENTS_CONTROLRegulating the substrate dosage bymeans of NO 3 -N measurementsTable 5: Average daily values of a municipal wastewater treatment plantAverage daily valuesInflow [m 3 /d] 10,000BOD 5 infl aer [mg/L] 110TN infl aer LATON [mg/L] 45P tot infl aer [mg/L] 3,5BOD 5 infl aer. : TN infl aer = 110:45 = 2.45Table 6: Calculation of amount of nitrogen that is not to be denitrified (ΣN n.d. )N incorporated in biomass (5 % of BOD 5 infl aer)N org.e (e = assumed target quantity in the outflow)NH 4 -N e (e = target quantity in the outflow)NO 3 -N e (e = target quantity in the outflow)SumTable 7: Denitrification capacity in accordance with ATV-A131(guideline values for dry weather and temperatures from 10 to 12°C)5.5 mg/L2 mg/L0 mg/L8 mg/L15.5 mg/LV D /V ATC Deni (Denitrification capacity in kg NO 3 -N D /kg BOD 5 )Volume Deni/Volume Aeration Upstream denitrification Simultaneous and intermittentdenitrification0.2 0.11 0.060.3 0.13 0.090.4 0.14 0.120.5 0.15 0.15V D : Volume of the aeration tank used for denitrificationV AT : Volume of the aeration tank3. Calculating the externalsubstrate requirementThe still to be denitrified N content is thetotal added nitrogen minus the amountof nitrogen that is not to be denitrifiedminus the amount of nitrogen that theplant can denitrify:S NO3-N, D, Ext = TN Inflow - N n.d. - S NO3-N, D= 45 mg/L - 15.5 mg/L - 16.5 mg/L= 13 mg/LTo denitrify the remaining 13 mg/L nitrogen,the microorganisms in the acti -vated sludge must be provided withan additional source of carbon. A dailywastewater volume of 10,000 m 3 has anitrogen load of 130 kg. According toDWA Work Sheet A131, the externalcarbon requirement is 5 kg COD/1 kgNO 3 -N. This means that, for completedenitrification to occur, 650 kg COD areneeded per day. If the additional carbonis provided in the form of acetic acid,the data provided in Table 8 indicate that607 kg would have to be added eachday. The targeted dosage is based onthe NO 3 -N values.Table 8: External carbon sources for calculating the necessary dosageAcetic acid Methanol EthanolCOD kg/kg 1.07 1.50 2.09TOC kg/kg 0.40 0.38 0.52BOD 5 kg/kg 0.70 0.96 1.35Density kg/m 3 1,060 790 780In this example, 1 kg acetic acid is equivalent to 1.07 kg COD.ConclusionsUnfavourable nutrient ratios and highconcentrations of individual substancesreduce the degradation efficiency ofbiological wastewater treatment processes.Early recognition and continuousmonitoring of critical parameters is thereforeessential in order to enable plantoperators to take rapid corrective actionwhen necessary. Only in this way cancompliance with legal outflow values beensured and unnecessarily high wastewaterlevies be avoided. LANGE cuvettetests and continuously operating processmeasurement devices have demonstratedthat they are indispensable aidsto achieving greater transparency andreliability.www.hach-lange.com