there are two processes for phosphorus removal, chemical phosphorus removal and biological phosphorus removal. biological phosphorus removal is a relatively economical method for phosphorus removal. from the most basic mechanism, main technological forms to in-depth discussion of the advantages and disadvantages of dosing methods and the calculation method of dosage of reagents, this paper comprehensively summarizes the knowledge points of chemical phosphorus removal from entry to proficiency.
1. phosphorus load in sewage
phosphorus produced by human food is unchanged, but at present, non-phosphorus detergent is widely used at home and abroad, so the phosphorus produced by detergent has decreased a lot in recent years. the concentration of phosphorus in raw water of municipal wastewater in china mainly depends on the phosphorus content in industrial wastewater. domestic sewage in foreign countries is generally 10-25 mg/l, while in china it is 5-10 mg/l. most of them are inorganic phosphorus compounds and are soluble. this part is mainly composed of orthophosphates and polycyclic phosphates from detergents. a small part of total phosphorus is organic phosphorus, which exists in dissolved and insoluble state. polycyclic phosphates (such as p3o105-) and organophosphorus compounds (nucleic acids) are generally converted into orthophosphates (po43-) in sewage networks and sewage treatment.
2. the basis of chemical phosphorus removal
chemical phosphorus removal is accomplished by chemical precipitation. chemical precipitation refers to the formation of granular and insoluble substances by mixing inorganic metal salts with soluble salts in sewage, such as phosphate. this process involves the so-called phase transfer process. for example, equation 1. in fact, after adding chemical agents, not only precipitation but also chemical flocculation are carried out in sewage. therefore, the difference between chemical precipitation and chemical flocculation must be distinguished (as shown in figure 1).
fecl3 k3po4_fepo4_ 3kcl(formula 1)
sewage sedimentation reaction can be simply understood as: dissolved substances in water, most of which are ionic substances converted to insoluble, granular form. flocculation is a process in which small, insoluble solids bind to each other to form larger shapes, so flocculation is not a phase transfer process.
in sewage purification process, flocculation and sedimentation are very important, but flocculation is used to improve the sedimentation effect of sedimentation tank, while sedimentation is used to remove dissolved phosphorus in sewage. if phase conversion is achieved by precipitation process, when soluble metal salts are added to wastewater, on the one hand, soluble phosphorus is converted into insoluble metal phosphate, and at the same time, insoluble hydroxides (depending on ph value) are produced. on the other hand, with the increase of precipitates and the accumulation of smaller insoluble solids into larger insoluble solids, the stable colloids are destabilized, and the destabilized colloids are contacted to form flocs through velocity gradient or diffusion process. finally, through the solid-liquid separation process, the purified sewage and the solid-liquid concentrate (chemical sludge) are obtained to achieve the purpose of chemical phosphorus removal.
3. types of chemical phosphorus removal agents
according to the basis of chemical precipitation reaction, in order to produce phosphate compounds, the chemical agents used for chemical phosphorus removal are mainly metal salt agents and calcium hydroxide (hydrated lime). when many high-valent metal ion agents are added to wastewater, they will combine with soluble phosphorus ions in wastewater to form insoluble compounds. for economic reasons, the main metal salts used for phosphorus precipitation are fe3 , al3 and fe2 salts and lime. these agents are used in the form of solution and suspension. divalent iron salts can only be used when the sewage contains oxygen and can be oxidized into trivalent iron salts. in practice, in order to be oxidized, fe2 is often added into aerated sand sink or synchronous precipitation process is used in aerated sand sink. the effect of fe2 is the same as that of fe3 , and the reaction formulas are as follows: formula 2 and 3.
al3 po43-alpo4_ph=6-7 (formula 2)
fe3 po43-fepo4_ph=5-5.5 (formula 3)
the reaction that competes with precipitation reaction is the reaction of metal ions with oh. therefore, attention should be paid to the amount of metal ions for various metal salt products, such as formula 4 and 5.
al3 3oh-al(oh)3(form 4)
fe3 3oh-fe(oh)3(formula 5)
metal hydroxides form large flocculants, which are beneficial to the flocculation of precipitated products, and also adsorb colloidal substances and fine suspended particles. it should be noted that the removal of organic matter by precipitation is secondary in the chemical precipitation reaction for the purpose of chemical phosphorus removal, but the coagulation of organic colloids and suspended matter in the floc is the decisive process in the separation.
the precipitation effect is affected by the ph value, and the solubility of metal phosphate is also affected by the ph value. for iron salts, the optimum ph value ranges from 5.0 to 5.5 and for aluminium salts from 6.0 to 7.0, because the solubility of fepo4 or aipo4 is the smallest within the above ph value range. in addition, the use of metal salts will also bring benefits to sewage and sludge treatment, such as reducing sludge index of sludge, which is conducive to biogas desulfurization.
the addition of metal salts can increase the content of cl-or so2-4 ions in effluent of sewage treatment plants. special attention should be paid to the presence of acid in the precipitating agent solution.
the alkalinity of sewage will be reduced by adding metal salts, which may have adverse effects on purification. when iron sulfate is used in synchronous precipitation process, the effect of iron sulfate on nitrification must be considered.
in addition, if sewage sludge is used in agriculture, aluminium or iron must be considered when phosphorus removal is carried out with metal salt reagent.
4. chemical precipitation process
chemical precipitation process is distinguished according to the place where the precipitating agent is added. in practice, pre-precipitation, synchronous precipitation and post-precipitation or flocculation and filtration are commonly used.
(1) pre precipitation
the characteristics of the pre-sedimentation process are that the sediment agent is added to the sedimentation tank, or to the intake channel (pipe) of the primary sedimentation tank, or to the venturi channel (using eddy current). it usually needs to set up devices to generate eddies or supply energy to meet the needs of mixing. the corresponding precipitation products (massive flocs) are separated by precipitation in a primary sedimentation tank. if the biofilter is used in the bio-section, the fe2 reagent is not allowed in order to prevent the damage to the filler (yellow rust).
the pre-precipitation process (as shown in figure 2) is particularly suitable for the renovation of existing sewage treatment plants (adding chemical phosphorus removal measures), because this process can not only remove phosphorus, but also reduce the load of biological treatment facilities. commonly used precipitation agents are quickash and metal salt agents. the content of residual phosphate after pre-precipitation is 1.5-2.5 mg/1, which fully meets the need of phosphorus for subsequent biological treatment.
(2) synchronized precipitation
synchronized precipitation is the most widely used chemical phosphorus removal process, accounting for about 50% of all chemical phosphorus removal processes abroad. the process is to add the precipitating agent to the effluent of the aeration tank or the inflow of the secondary sedimentation tank. in some cases, the precipitating agent is also added to the inflow of the aeration tank or the reflux sludge canal (pipe).
fig. 3 is a process sketch of activated sludge process with synchronous precipitation. when using biological rotary disc process, the situation is similar to that of activated sludge process, but it is still worth discussing whether biological filter process can add chemicals to the influent of secondary sedimentation tank.
(3) back analysis
post-sedimentation is the separation of sedimentation, flocculation and flocculated substances in a facility separated from biological facilities, so there is a two-stage process. generally, precipitating agents are added to a mixing tank (m tank) after the secondary sedimentation tank, followed by flocculation tank (f tank) and sedimentation tank (or air floatation tank).
the sketch of post-precipitation process is shown in fig. 4. for the water receiving water which is not strict, the lime emulsion agent can be used in the subsequent precipitation process, but the ph value of the effluent must be controlled, for example, the co2 in the biogas is used for neutralization.
air flotation tank can remove suspended solids and total phosphorus better than sedimentation tank, but the operation cost is higher because of the constant supply of air.
the advantages and disadvantages of the three processes are summarized in table 3.
5. calculation of chemical precipitation agent quantity
the removal of one molecule of phosphate from formula 2 and 3 requires a molecule of iron or aluminium salts. for the convenience of calculation, mol or atomic weight is used in practical calculation.
when removing phosphorus by chemical precipitation, the removal of lmol (31g) p requires at least lmol (56g) fe, or at least 1.8 (56/31) times fe, or o.9 (27/31) times al. that is to say, removing lgp requires at least 1.8g of fe, or 0.9g of al.
in practice, the reaction is not 100% effective, and oh - will compete with metal ions to produce corresponding hydrogen oxidation, such as formula 4 and formula 5. therefore, the actual chemical precipitation agent dosage generally needs to be overdosed to ensure the required effluent p concentration. in german calculation, the concept of input coefficient beta is put forward.
beta=(molfe, molal)/molp(formula 8)
addition coefficient beta is affected by many factors, such as the place of adding and mixing conditions. it is suggested that the actual addition should be determined by adding experiments. fig. 5 is the relationship between the addition coefficient and the amount of phosphorus reduction. under the optimum conditions (suitable dosage, good mixing and flocculation conditions), beta = 1; under the non-optimum conditions, beta = 2 to 3 or higher. excessive dosage of reagents will not only increase the cost of reagents, but also greatly increase the amount of sludge due to the formation of a large number of hydroxides, which is large and difficult to dehydrate.
in germany, in order to effectively remove phosphorus (the effluent remains less than 1 mg p/1), the beta value is 1.5, that is to say, to remove 1 kg of phosphorus, it is necessary to add:
1.5 x(56/31)=2.7 kg fe
or,
1.5 x(27/31)=1.3 kg al
if lime is used as chemical precipitation agent, this calculation method can not be used, because the ph value of lime is more than 8.5, and the dosage is affected by the alkalinity of sewage (buffer capacity), so the dosage must be determined by experiments for each sewage.
strictly speaking, the concept of adding coefficient beta only applies to post-precipitation. for pre-precipitation and synchronous precipitation, we should also consider: 1) unreacted chemicals are contained in reflux sludge; 2) phosphorus removed in primary sedimentation tank and biological process.
6. examples of calculation
example 1: the designed water volume of the sewage treatment plant is 10 000 m3/d, the concentration of p in the influent is 14 mg/1, and the concentration of p in the effluent is 1 mg/l. alcl3, a precipitating agent, was designed. its active ingredient was 6% (60g/kg alcl3) and its density was 1.3kg/l. in order to synchronize precipitation, the dosage needed was calculated.
solution:
after the first precipitation, the phosphorus removal rate was 2 mg/l, the p concentration in the influent of the biological treatment facility was 11 mg/l, and the p removal rate by biological assimilation was 1 mg/l. for those that need to be removed by precipitation:
p load =
10000m3/d. (0.011-0.001) kg/m3 = 100kg/d
the design adopted the input coefficient beta value of 1.5.
the dosage of design al is as follows:
1.5 x (27/31)x 100 = 130kg al/d
conversion required dosage:
130*1000(g/d)/60(g/kg)=2167 kg/dalcl3
conversion requires volume:
2167(kg/d)/1.3(kg/l)=16671/dalcl3
case 2: feso4 sulphate was used as a drug. the effective component was 180 gfe/kg feso4. the saturated solubility was 400 gfeso4/l at 10 c. the other design parameters were the same as that of case 1.
solution:
the design adopted the input coefficient beta value of 1.5.
the dosage of designed fe is as follows:
1.5 x 5631 x 100 = 270kg fe/d
conversion required dosage:
270*1000(g/d)/180(g/kg)=1500 kg/dfeso4
the effective components in saturated solution are:
180(g/kg). 0.4(kg/l)=72 gfe/l feso4
conversion requires volume:
1500.1000(g/d)/72(g/l)=20833l/dfeso4
7. effect of sedimentation on sewage treatment
(1) effect of precipitation on metal content in effluent of sewage treatment plant
the content of metals and reagents in effluent of sewage treatment plant mainly depends on the separation of suspended solids. of course, the dosage of reagents, beta value, ph value, alkalinity of sewage and the dosage technology also have an impact on them.
iron and aluminium in effluent of sewage treatment plant are generally insoluble phosphate and hydroxide, which exist in suspension state.
in the case of normal dosage of reagent (e.g. beta=1.5, synchronous precipitation), neutral ph and sufficient secondary sedimentation tank or sedimentation tank, the content of al and fe will not exceed 1.0 mg/l, although the iron content in the influent of sewage treatment plant often exceeds 1.0 mg/l; for the effluent of flocculation filter, the content of fe or al is generally less than 0.5 mg/l.
(2) effect of precipitation on salt content in effluent
phosphorus precipitation with metal reagents will inevitably lead to an increase in salt (cl-or so42-content) in effluent from sewage treatment plants. its increment can be determined by calculation:
for example, in case 1, alcl3 was added because 1 kg of al corresponded to 3.9 kg of cl-, cl-, which increased by 3.9 kg (3 *35.5/27=3.94):
130*3.94=513 kg cl-/d
the converted concentration is:
513kg/d *1000/10000m3/d = 51.3mg/l
in case 2, feso4 was added, because 1 kg fe corresponded to 1.7 kg so42-(32 4*16)/56=1.71) kg, the increase of so24-:
270*1.7=461.7 kg so24-/d
the converted concentration is:
461.7kg/d *1000/10000m3/d = 46.2mg/l
cl-<100mg/l and so24-<200mg/l in the normal effluent of municipal wastewater treatment plant containing a small amount of industrial wastewater, that is to say, the chloride content in effluent increased by 50% and so42-increased by 25% after using metal agents. when there is a strict requirement for the accepted water body, the salt content should be checked.
because bicarbonate is removed at the same time as phosphate precipitation, the total salt content (conductivity) in effluent remains almost unchanged.
(3) effect of precipitation on alkalinity
alkalinity of water refers to the amount of hci used to make a liter of water reach a certain ph value. alkalinity also refers to the buffer capacity of acid. the alkalinity of the influent of sewage treatment plant corresponds to the alkalinity of the drinking water in its basin and the alkalinity produced by ammonium. in phosphate precipitation, as long as iron or aluminium ions enter the aqueous solution, a hexahydrate complex is formed. the general form is me (h_2o) 3 6 (me: metal), which can be further hydrolyzed like acid:
me(h_2o)63 3h me(oh)3 3h_2o(form 9)
the reaction is related to the ph value of the solution and reduces the alkalinity of the water. because the precipitation of hydroxides in the form of insoluble complexes will not increase the alkalinity of sewage, the acid equivalent must be estimated for the precipitation of metal hydroxides, as well as for the precipitation of metal phosphates. separation of phosphate from synchronous precipitation can only slightly improve the alkalinity of wastewater.
(4) effect of sedimentation on the production of surplus sludge
as mentioned above, the removal of dissolved phosphorus in sewage results in sludge production, and the removal location of sludge varies with different processes. for synchronous precipitation, it is discharged in the form of excess sludge. excess sludge yield is an important parameter in sludge treatment design and operation. when chemical phosphorus removal is carried out by simultaneous precipitation, unit sludge yield is composed of excess sludge produced by removal of bod5 and precipitates of simultaneous precipitation phosphorus removal.
for the sludge produced by simultaneous precipitation, the type of precipitating agent and the molar ratio of metal ions to precipitated phosphorus are used to determine the sludge produced by chemical phosphorus removal. when beta=1.5, 1 kg fe was added to produce 2.5 kg of dry matter, or 1 kg al was added to produce 4 kg of dry matter.
(5) effect of precipitation on nitrification
in the case of simultaneous precipitation with ferric sulfate reagent, the nitrification reaction is hindered. in this case, it is recommended that the sludge age be increased by 10%. ferric chloride has no effect on nitrification. table 4 shows the influence coefficients of various precipitation processes on nitrification. the influence coefficients are the ratio of sludge age under specific process conditions to that under conventional process conditions (without phosphorus removal and with the same nitrification capacity).
because insoluble carbons can also be precipitated at the same time as pre-precipitation, which can not provide enough carbons for denitrification required for the stabilization of nitrogen oxidation process, pre-precipitation also has a negative effect on nitrogen removal. the common problem is that many carbon compounds have been removed by one precipitation, which is often insufficient for the pre-denitrification reaction, and the pre-precipitation aggravates this contradiction.
source: polaris environmental protection network
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