Acid Production And Optimization Of Waste Activated Sludge Fermentation Under Alkaline Condition

Overload of nitrogen and phosphorus in the waterbody is the main reason foreutrophication, an essential strategy for controlling eutrophication is to removenitrogen and phosphorus from wastewater before discharge. Activated sludge processis widely used for nitrogen and phosphorus removal home and abroad in the wastewater treatment plants （WWTPs）. Readily degradable carbon sources, such as shortchain fatty acids （SCFAs） are need in the biological nitrogen and phosphorus removalprocess. However, carbon sources are often not adequate in the influent.Simultaneously, a large amount of waste activated sludge production in the activatedsludge process became another problem that should be solved urgently. Therefore,technology which both can produce carbon sources in the WWTPs and realize sludgereduction is needed.Under alkaline condition, abundant VFA could be produced in the fermentationprocess of waste activated sludge （WAS）, and the reduction rate of sludge could beenhanced compared with those under neutral and acid condition, hence fermentingWAS under alkaline condition can solve the two problems mentioned above. Theinfluence factors, mechanisms and microorganisms of WAS alkaline fermentationprocess had been studied by researchers. However, which alkali and sludgeconcentration are the best, how to further improve the acid production of the WASalkaline fermentation process, how to improve the sludge dewaterability of WAS inthe alkaline fermentation process and how to recycle carbon source, nitrogen andphosphorus with a low cost still need to be discussed. A series of batch tests weredone by treating WAS under alkaline conditions to give answers to those problems.Effects of alkali types and sludge concentrations on WAS alkaline fermentationwere investigated to indentify which alkali and sludge concentration are the best.Alkali types （KOH、NaOH、Ca（OH）2and Na₂CO₃） influenced significantly on WAShydrolysis, acidification and dewaterability. WAS hydrolysis was the best underNa₂CO₃condition, and sludge acidification was the best but the sludge dewaterabilityis the worst under NaOH condition, the sludge dewaterability is the best but sludgeacidification was the worst under Ca（OH）2condition. Under15℃and35℃, sludgeconcentrations （volatile suspended sludge concentrations were1.708、3.146、5.124、6.832、8.540and11.049g/L, respectively） insignificantly influenced sludge hydrolysis, but high and low sludge concentrations disadvantaged VFA production of WAS, andthe best sludge concentration for acid production was8.540g/L.Effects of fermentation mode, adding magnesium powder on sludge hydrolysisand acidification in WAS alkaline fermentation process, and the effects of hydroxyland metal ions in the alkaline fermentation process were investigated to furtherimprove the acid production of the WAS alkaline fermentation process. The effects ofpH values on mixed sludge （mixture of primary sludge and waste activated sludge）hydrolysis and acidification were firstly studied, then hydrolysis and acidificationabilities of separately-fermented sludge and co-fermented sludge were compared. Theresults showed that the effects of pH on hydrolysis and acidification of primary sludge,waste activated sludge and mixed sludge were similar, alkaline conditions benefitedthe hydrolysis and acidification of the three types of sludge, and the best pH value foracid production were all at pH10under20²2℃. Compared withseparately-fermenting primary sludge and waste activated sludge, co-fermenting themdidn’t impede acid production but could improve acid production. Adding magnesiumpowder didn’t affect WAS solubilisation, but can enhance the activity of protease andimprove VFA production, and the optimal addition was0.01g/g （TSS）.Theenhancement of NaOH on solubilisation and acidification of WAS were mainly due tohydroxyl, and partly due to sodium ion. NaCl improved the solubilisation of WAS,and sludge solubilisation efficiencies, protein and carbohydrate release all increasedwith the dosage of NaCl, NaCl also could inhibit the growth of methanogens. Theimprovement of sludge solubilisation and inhibit of the growth of methanogens byNaCl enhanced VFA production of WAS.Fermentation liquid should be separated from fermented sludge afterfermentation to be used as carbon source for nitrogen and phosphorus removal.Hydrolysis, acidification and dewaterability of waste activated sludge （WAS） wereinvestigated at pH10controlled by the addition of NaOH, Ca（OH）2or their mixturesat various ratios. Under the solo NaOH condition, VFA production was high. However,the sludge dewaterability was negatively affected due to a large release of Na+fromNaOH. Under the Ca（OH）2condition, the sludge dewaterability was good due to theaddition of Ca（OH）2, but the VFA production was low. Because Ca²⁺was releasedthrough neutralizing reaction between VFA and Ca（OH）2, and a high concentration ofCa²⁺inhibited the hydrolysis of protein, so only a small quantity of VFA could beproduced. Under the mixed alkaline condition, the concentrations of Na+in the fermentation liquid decreased and Ca（OH）2was provided simultaneously, both ofwhich could improve sludge dewaterability. Ca²⁺released by Ca（OH）2could beprecipitated by reacting with NaOH and CO2, and little amount of Ca²⁺could bereleased from Ca（OH）2. The low concentration of Ca²⁺insignificantly inhibited thehydrolysis of protein, thus allowing a high level of VFA production. Thus, treatingWAS with mixtures of NaOH and Ca（OH）2could enable both high VFA productionand good sludge dewaterability.A significant amount of NH₄⁺-N and PO₄^3--P was released into the fermentationliquid in the fermentation process of WAS, which could impede fermentation liquid tobe used as carbon source for nitrogen and phosphorus removal. NH4+-N and PO43--Pshould be removed firstly before the fermentation liquid was used for nitrogen andphosphorus removal. The hydrolysis and acidification of waste activated sludge wereinvestigated under three types of alkaline conditions, which were NaOH, Na3PO4andcombination of NaOH and Na3PO4. Then the recovery of ammonia and phosphorus inthe form of struvite from the three types of alkaline fermentation liquid was conducted.The results show that the hydrolysis and acidification abilities of WAS under the threetypes of alkaline conditions were similar. However, the molar ratio of phosphorus andammonia in the sludge fermentation liquid was in the range of0.9¹.4under themixed alkaline condition, which was the most suitable for ammonia and phosphorusrecovery in the form of struvite. Under the mixed alkaline condition, the dosage ofNaOH in the alkaline fermentation process could be saved, and the dosage of HCl alsocould be saved in the nitrogen and phosphorus recovery process, so combined use ofNaOH and Na3PO4could save the cost for recycling carbon source, nitrogen andphosphorus.