The Effect on Activated Sludge of Chemical Coagulants Applied in Synchronization Dephosphorization ()
1. Introduction
With discharge standard for municipal wastewater treatment plant becoming stricter, 1A discharge standard in Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002) has been extensively executed in China. Traditional biological dephosphorization technology cannot meet the new discharge standard any more and chemical coagulation method is added into upgraded and improved technology to assist dephosphorization [1]. As for wastewater treatment plants that have not been upgraded and improved, it is also an important treatment countermeasure to intensify dephosphorization by adding coagulants into biochemical system [2]. Then coagulant has two effects on microorganism: the first one is biological inhibition of direct addition on biochemical system; the other one comes to indirect influence of chemical sludge and residual coagulant cycle formed by advanced treatment on biochemical system. The inhibition influence of coagulant on microorganism has already drawn people’s attention [3,4], and its influence degree is related to the applied treatment process, dosing method and coagulant type.
Coagulant of PAC (polyaluminium chloride) is adopted by North sewage treatment plant (North STP) in the city of Suzhou to assist dephosphorization. Two kinds of dosing methods were used, with one adding PAC to biochemical system before finally clarifier and the other one adding PAC to outflow after clarifier promoting coagulating in static mixer. There is no coagulating sedimentation tank before sand filter, which results in the fact that backwash wastewater from sand filter directly discharges into inlet pumping station and finally chemical sludge in backwash wastewater circulates into biochemical system. In order to verify and evaluate effects of coagulant on biochemical system, the influences of coagulant type and dosage on microorganism are studied and effects of longterm cycle of chemical sludge in biochemical system on microorganism are analyzed in this paper.
2. Material and Method
2.1. Technical Flowchart
AAO biological treatment process is adopted by North STP with a scale of 100,000 m3/d. Meanwhile 1A discharge standard is executed, in which total phosphorus of outflow is below 0.5 mg/L. The technical flowchart is shown in Figure 1.
Figure 1. Technical flowchart of the North STP in Suzhou.
2.2. Measurement for Oxygen Uptake Rate
Fresh activated sludge is taken from the middle part of biochemical pool, coagulant (PAC, AlCl3, Fe2(SO4)3) or backwash wastewater is added in different concentration ratios (10 - 35 ppm), and the oxygen uptake rate (OUR) of microorganism in the sewage plant is measured rapidly. In order to maintain consistency of measurement environment, one blank contrast is taken from the same batch for simultaneous measurement, with reaction bulb and stirrer unchanged. The same brand and model should be used for dissolved oxygen measurement. The dissolved oxygen value in reactor reduces from DO1 to DO2 as the extension of time. The OUR is calculated as: OUR = (DO1 − DO2)/Time.
2.3. Measurement Methods for Key Indexes
Analysis of water conventional index refers to Water and Wastewater Monitoring Analysis Method (Version 4). Inductively Coupled Plasma Atomic Emission Spectrometry is adopted for total aluminum (Al) analysis in water. Total aluminum (Al) analysis in sludge refers to General Rule for Inductively Coupled Plasma Atomic Emission Spectrometry (JY/T015-1996). The VSS/SS ratio means volatile suspended sludge concentration to suspended sludge concentration.
3. Result and Discussion
3.1. Effect of Aluminum Salt Coagulant on Microorganism Activity
Considering Aluminum salt is the most common coagulants, AlCl3 and PAC are firstly selected to measure the OUR change of microorganism. In order to evaluate the direct influence of different coagulants on microorganism activity, effects of AlCl3 and PAC on microorganism activity are presented in Figures 2 and 3, according to different addition concentrations (calculated in Al).
The figures show that a good linear relation is presented in change of OUR. The higher concentration of dosage has greater effect on OUR. When AlCl3 dosage is 15 ppm and 35 ppm, OUR reduces from 0.582 mg/ (L·min) of blank sample to 0.487 mg/(L·min) and 0.305
Figure 2. Effect of AlCl3 on microorganism activity.
Figure 3. Effect of PAC on microorganism activity.
mg/(L·min) respectively, with the average inhibition rates of 16.1% and 45.7% separately. When PAC dosage is 10 ppm, 20 ppm and 30 ppm, the average inhibition rates are 11.9%, 33.0% and 80.1% respectively. Therefore, Aluminum salt coagulant has direct inhibition effect on microorganism. The inhibition effect of PAC is 3 times more than that of AlCl3, and coagulant with higher concentration dosage also has higher inhibition rate on microorganism. It is generally recommended that during direct application to biochemical pool for dephosphorization, the dosage should be less than 10 ppm, so as to decrease the inhibition effect as far as possible.
3.2. Effect of Iron Salt Coagulant on Microorganism Activity
Although the precipitate composed of iron salt and phosphate is more stable and has better effect when compared with aluminum salt, its application alone is restricted owing to its characteristics of tolerating pH and coloring outlet water. However, coagulant composed of aluminum and iron has attracted much attention and gained extensive application [5]. In order to select the best dephosphorization coagulant, contrast test is conducted for the effect on microorganism activity by using Fe2(SO4)3 and AlCl3 under the concentration of 10 - 20 ppm, calculated in aluminum or iron ions.
Figure 4 shows that when AlCl3 and Fe2(SO4)3 are added in a concentration of 10 ppm, OUR of microorganism reduces from 0.575 mg/(L·min) of blank sample to 0.446 mg/(L·min) and 0.499 mg/(L·min) respectively. Inhibition rate by AlCl3 is 9.6%, compared with 5.4% by Fe2(SO4)3. When AlCl3 and Fe2(SO4)3 in a concentration of 20 ppm are added, OUR of microorganism reduces from 0.69 mg/(L·min) of blank sample to 0.475 mg/ (L·min) and 0.576 mg/(L·min) respectively as shown in Figure 5, with the inhibition rates by AlCl3 and Fe2(SO4)3 of 15.8% and 8.5% separately. Therefore, inhibition rate to microorganism by Fe2(SO4)3 is far lower than that by AlCl3. When the dosage is 10ppm, inhibition effects of both aluminum salt and iron salt are small, with the inhibition rates of 9.6% and 5.4% respectively. In order to reduce effects of coagulant on biochemical system, iron salt, rather than aluminum salt, should be adopted when coagulant is directly added into biochemical system, and the dosage should be less than 20 ppm.