Magnesium Ammonium Phosphate Method For Nitrogen And Phosphorus Recovery In Anaerobic Digestate And Its Process Research And Grayscale Analysis

Anaerobic digestion produces biomethane,which can be used as bioenergy.Anaerobic slurry contains a large amount of organic matter and high concentrations of ammonia nitrogen and total phosphorus.This paper aims to study the low-cost magnesium ammonium phosphate（MAP）method to recover high-concentration N and P nutrients in anaerobic digestion slurry.The complex water quality conditions of anaerobic digestion slurry,optimization of parameters,realization of precipitation time and precipitation separation are main influencing factors restricting the application of this technology.The effects of different p H values,P/N ratios and Mg/N ratios on the removal efficiency of ammonia nitrogen and total phosphorus in anaerobic digestion slurry treated by MAP method were studied.When the p H value is 9.5,the P/N ratio is 1.00,and the Mg/N ratio is1.05,the effect of simultaneous recovery of ammonia nitrogen and total phosphorus is the best.The MAP method was used to remove and recover ammonia nitrogen and total phosphorus in reactor anaerobic digestion solution,agricultural anaerobic digestion solution,high-phosphorus industrial wastewater,and livestock and poultry wastewater,and a high recovery rate was obtained.The ammonia nitrogen in the anaerobic digestion slurry decreased from 2459 mg/L to 50.4 mg/L,the total phosphorus decreased from 579 mg/L to5.5 mg/L,and the NH₄⁺-N recovery rates reached 97.95%and TP 99.05%,respectively.The solid phase analysis by SEM and EDS was carried out.Based on the MAP method to recover nitrogen and phosphorus from anaerobic digestion slurry,the MAP-MINTEQ model was constructed and optimized.The chemical equilibrium model Visual MINTEQ 3.1 was used in combination with experiments for modeling,and the model parameters with high fitting degree were obtained.Firstly,the influence of the inherent carbonate in the anaerobic digestion solution on the reaction of ammonium magnesium phosphate was studied,and the influencing factors and correction coefficients of the activity coefficientγin the two activity coefficient equations were compared.The extended Debye-Hückel Equation fitted the MAP-MINTEQ model better.Secondly,the effect of the complex acid-base buffer system in the biogas slurry on the precipitation and dissolution equilibrium of the by-product Mg CO₃ was studied,and the MAP-MINTEQ model was used to optimize the fitting results with the calibrated k_sp.Based on the optimized model,the experimental scale of the MAP reaction was adjusted to achieve increased nitrogen and phosphorus recovery.According to the calibration MAP-MINTEQ model,the residual ammonia nitrogen concentration dropped to 41.2 mg/L,and the nitrogen recovery rate reached 98.0%.The minimum total phosphorus is 5.5mg/L,and the recovery rate of phosphorus is 99.2%.In this study,the influence of the sedimentation performance of MAP particles was studied,and the relationship between the sedimentation rate of short rod-shaped crystal particles of MAP with the precipitation additives,reactant ratio,and process control was discussed in Stokes Equation.In order to increase the sedimentation rate,Ca Cl₂ and microcrystalline cellulose were added as precipitation aids to improve the sedimentation efficiency and assist the MAP precipitation and separation process.The turbidity of the simulated wastewater supernatant was reduced to 7.9 NTU,achieving 35.0%COD_cr and 46.9%turbidity removal of Ca Cl₂ in the anaerobic digestion slurry.The precipitation time can be shortened to 37.7%by adding microcrystalline cellulose,and the optimum concentration is2 g/L.Based on a self-designed machine vision monitoring reactor,the MAP precipitation process of anaerobic digestion slurry was studied.Using the image processing function of Open CV-Python,the clarified phase,precipitated phase and interface layer were automatically identified,the sedimentation state was judged,the drainage control is fulfilled then.Image processing includes reading video frame images,grayscale processing,Gaussian blur,binarization processing,morphological operations,and drainage logic judgment.The relationship between the depth of clarified phase,the gray level of interface layer,the average gray scale of precipitated phase and the average gray scale of clarified phase with the development of precipitation time was studied by using machine vision to monitor the reactor.The depth of clarified phase,the gray value of precipitated phase,and the gray value curve of clarified phase all conformed to the law of logarithmic function log3P1,and the fitting degree was good.With the development of precipitation time,the gray value of the clear phase showed a downward trend.The gray value of the precipitated phase changes dynamically with the development of sedimentation time and pixel depth,and is affected by the gray value of the clear phase,the depth of the interface layer,and the size of precipitation particles.The visual monitoring of the sedimentation reactor established a rapid determination method for recognition the clarified phase and the precipitated phase by using the change of the gray value,which avoids the extensive separation of the regular slag discharge in the traditional sedimentation tank,improves the solid-liquid separation effect,and improves the utilization of the reactor space.