Optimal nutrient ratios for wastewater treatment - HACH LANGE

4 NUTRIENTS_CALCULATIONFavourable and unfavourablenutrient ratiosSC 1000 controller for up to eightsensors – also suitable for useas a network with, for example,ProfibusHowever, all sorts of industrial plants,regional differences in eating habits (disposalof different kitchen wastes throughthe drains), and the nature of the soil anddrinking water cause wastewater to varywidely in its composition. Experience hasshown that the C:N:P ratio in municipalwastewater is about 100:20:5.The excess N and P compounds canusually be eliminated from the wastewaterwithout any great difficulty usingmodern methods.If the wastewater in the inflow to the biologicalstage is deficient in one of themain nutrients, a wide range of problemsmay occur (Table 3).For efficient denitrification, a certainproportion of readily biodegradable Ccompounds must be present. Aftermunicipal wastewater has passedthrough the primary settling tank, it has aBOD 5 :N ratio of 100:25 (=5). If the ratiofalls below 100:40 (=2.5), the efficiencyof the denitrification process is impaired,resulting in higher nitrate values in theoutflow. If bypassing the primary treatmentand increasing the denitrificationvolume fail to bring about any improve-Partially immersed NITRATAX scprobe for online nitratemeasurementTable 2: Causes and effects of high sulphur concentrationsCauses/Origin of wastewater Possible consequences Corrective action• High concentrations of sulphurcompounds from chemicaland protein processingindustries (meat and poultryprocessing)• Anaerobic processes in thesewerage system, whichcause sulphur compoundsto be reduced to hydrogensulphide• Corrosion in sewers andtank walls in wastewatertreatment plants• Neighbours suffer odournuisance• Increased growth ofsulphur oxidising filamentousbacteria (Type 021 N)• Avoid blockages in thesewerage network• Add iron salts to the sewer(e.g. at the pumping stations)Table 3: Causes and effects of nutrient deficiencies in the biological stage of wastewater treatmentShortage of Causes/Origin of the wastewater Possible consequences Corrective actionCarbon • Long dwell time in the seweragenetwork• Far-reaching primary treatment ofthe wastewater• Industrial wastewater with a highnitrogen content, e.g. from milk andmeat processing• Profuse development of filamentousbacteria (sludge bulking and foam)• Insufficient denitrificationNitrogenPhosphorusLow-nitrogen wastewater from:• Paper industry• Fruit and vegetable processing• Landfill leachate, wastewater fromfruit and vegetable processing• High COD/TOC values in the inflowof the wastewater treatment plant• Filamentous bacteria• Increased COD/TOC values in theoutflow• Filamentous bacteria• Bypass the primary treatment• Increase the denitrification volume whileretaining sufficient volume for the nitrification(minimum sludge age of 9 days)Balance the nutrient ratio by:• Addition of N compounds(good-value industrial products such as urea)• Addition of domestic wastewater, turbid waterfrom digesterBalance the nutrient ratio by:• Addition of P compounds (good-value industrialproducts such as phosphoric acid or phosphatefertilisers for the agricultural sector)• Addition of domestic wastewaterwww.hach-lange.com