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|Title:||Investigations on Nitrification Process of Ammonium-Rich Wastewater in Single Stage Activated Sludge Systems|
|Keywords:||Water - Purification - Palestine|
industrial wastewater, nitrogen-rich, nitrification, denitrification
Sewage - Purification - Nutrient removal - Palestine
Factory and trade waste - Management - Palestine
Sewage - Purification - Nitrogen removal - Palestine Nitrification. Sewage -- Purification -- Activated sludge process.
Nitrification - Palestine
Sewage - Purification - Activated sludge process - Palestine
|Citation:||Al-Sa`ed, R., 1987. Investigations on Nitrification Process of Ammonium-Rich Wastewater in Single Stage Activated Sludge Systems. Dissertation (Dr.-Ing.), Faculty of Civil Engineering, TU Braunschweig, Braunschweig, Germany|
|Series/Report no.:||Publications of The SiWaWi Institute Heft 43;|
|Abstract:||Extended Summary: The biological nitrification process of high strength ammonium with low BOD wastewaters was investigated in completely mixed, continuous and single-stage activated sludge systems. Five identical bench scale units were operated in parallel under steady state conditions. Four reactors (A, B, C, and D) were fed with a synthetic wastewater while the fifth (reactor E) was fed with industrial effluents of a steel work plant. A summary of the major results obtained from different experimental phases are presented in this section together with conclusions. The effect of C/N ratio an nitrifiers fraction (Fn) and nitrification rates (qn and qoTSn) was studied during phase one over a period of 200 days. It was found that these variables (Fn, qn and qoTSo) increased inversely proportional to C/N ratio. Reactor A (C/N ratio = 0,75:1) showed approximately 67% less than reactor D (C/N ratio = 0,08:1) for Fn value at an verage sludge age of 10 days. At sludge ages (tTS) greater than 10 days, almost constant nitrifiers fraction (Fn) , biomass (Xvn) and volumetric nitrification rates (qn) were noticed in all reactors. However, at tTSlower than 10 days a drastic decrease in Fn, Xvn and qn has been observed. Therefore, an increase in nitrifiers biomass above the critical sludge age did not increase volumetric nitrification rates at high sludge ages. Below the critical sludge age, the nitrification stability was found to be highly dependent of C/N ratio - sludge age interaction. With increasing sludge wastage i.e, decrease in tTS value, an increase in both BOD and NH4-N sludge loading rates (BoTSc and BoTSn) has been observed. Reactor A (C/N = 0,75) showed at a BoTSn of 0,25 kg N/kg oTS*d an increase (5 mgN/l) in effluent NH4 and O2 concentration. On the other hand, reactors B, C, and D at lower C/N ratios but at the Same tTS (10 days) revealed the same effluent concentrations (5 mgN/l) at high BoTSn (0,45 kgN/kgoTS*d). This would emphasise the effect of C/N ratio-sludge age interaction an the activity of nitrifiers. A synergestic effect resulted from pH, decrease in sludge age (high BoTSc and BoTSn) and increase in NH4 and NO2 concentration in the bulk of the reactor simulated the build-up of free ammonia (FA) and free nitrous acid (FNA). High concentrations of FA (0,1 to 83 mg/l) and FNA (0,02 mg/l) were measured at low sludge ages (tTS smaller than 10 days) and BoTSn (above 0,45 kgN/kgoTS*d) causing a decrease in both Xvn and qn. Sludge settling characteristics were found to be a function of C/N ratio and sludge age. Extensive filamentous growth of Nocardia and Sphaerotilus natans was observed in reactor A, B, and C but not D. This was due to sludge ages below 10 days but not due to nutrient limitation or inconsistancy in wastewater (P/N ratio) composition. This poor sludge settling behaviour might enhance the nitrification instability. Nitrifiers kinetic data analysis revealed low average values of nitrifiers growth rates (UnmT= 0,15 d -1). Linearization of Michaelis-Menten equation was utilized to determine UnmT and the NH4 saturation constant ( Kn) . Any attempt made to obtain nitrifiers kinetic constants from nitrification rates expressed per total organic (qoTSn) matter per day, resulted in erroneous data. The minimum sludge age (tTsm) was always lower than the operating sludge age (tTS) above the critical sludge age (tTS = 10 d). However, at a sludge age of 7 days, a safety factor of 1,45 should be used to achieve complete nitrification. Carbonaceous (BOD) removal rates (qoTSc) were independent of influent BOD concentration, i.e, zero-order reaction similar to NH4 oxidation. Reactor A had the highest heterotrophic biomass (Xvh) because the later increased with increasing C/N ratio. No significant effects of BOD sludge loading on nitrification process were of BOD sludge loading on nitrification process were observed at BoTSc below 0,15 kgBOD5/kgoTS*d). However, with decreasing sludge age (tTS below 10 days) and at higher C/N ratios > 0,16:1), bulking sludge might dominate and consequently exerts negative effects on nitrification. Spiking the Feed with huge amounts of NH4Cl to reach the required C/N ratios, Cl-concentrations ranged in the bulk of the reactors (A, B, C and D) from 200 to 6000 mg Cl-/l. Such relatively high chloride concentrations might have been attributable to inhibitory effects especially at low sludge ages. This type of inhibition can be explained on the one hand, partially based an physiological aspects (osmotic shock) causing cell lysis and finally nitrifiers decay. On the other hand, thermodynamic calculations revealed that nitrifiers were able to utilize only 5 to 15 % of the free energy released during nitrification for metabolism and cell synthesis. The rest of free energy is lost just because of inter- and intracellular balance of solute active transport in- and outside the cell. Thus, inhibitory effects caused by high Cl- concentration and low energy gained for cell synthesis enhanced the instability and process failure of nitrification. Increasing the hydraulic loading (qR) during phase two, i.e, decreasing the hydraulic retention time (HRT), caused no significant effects on nitrification. Further, increasing the NH4 volumetric loading (BRn) revealed an increase in nitrification rates and nitrifiers biomass in all reactors at an almost constant sludge age (tTS= 23 days). Linear relationship was observed between NH4 loading and nitrification rates (zero-order reaction). Approximately 95% NH4-N was removed at tTS= 23 days and volumetric NH4loading of 0,46 kgN/m3*d. The minimum and maximum values of HRT varied between 0,24 day (reactor A) and 2,85 days (reactor D) respectively. At relatively high sludge age (tTS= 20 days) and low hydraulic retention time, carbonaceous sludge sludge loading might be limiting and enhanced sludge bulking. This was observed at C/N ratio (C/N below 1:1) and BOD sludge loading (BdTSc below 0,1 kgBOD/kgoTS*d). As a result partial nitrification was noticed in reactor B, C and reactor D. This would suggest that presence of organic matter might stimulate the nitrifiers activity by providing a compact sludge floc formation. On the contrary pinpoint floc structure or bulky sludge might enhance the washout of nitrifiers from the system. At high NH4sludge loading (BoTSn above 0,3 kgN/kgoTS*d) and small hydraulic loading might cause process instability. Free ammonia and free nitrous acid inhibitory effects might prevail under such process conditions and cause an increase in effluent NH4 and NO2concentrations. Batch respirometric methods applied were not successful in nitrifier’s biokinetic determinations in mixed cultures. Attempts performed in this study to utilize nitrifiers oxygen uptake rates (OURn) revealed erroneous data and poor point distribution. This might be due to several reasons. It was observed that nitrifiers inhibitory substances such as allylthiourea (ATU) and N serve were not inhibitory to Nitrobacteria activity. Results of some authors indicated that ATU inhibited the endogenous metabolism of both auto- and heterotrophs. Further, whether heterotrophic nitrifiers, which might be present in the sludge, are affected by ATU, is still unknown. However, this method can be applied to determine the total activity of the sludge and not for specific nitrifiers constants. On the contrary, NH4-N oxidation rates both in batch and continuous operation utilized for nitrifiers growth constants determinations showed satisfactory results. The volumetric nitrification rates (qn) as mgN/l*h were substituted in Monod equation and in the linearized form of Michaelis-Menten equation to determine the growth coefficients. The nitrifiers biomass, activity and growth rates were found to be a function of reactor temperature. Reactors temperature, varied between 15 °C and 25 °C, caused an increase (10%) per 1°C temperature rise. Furthermore, it was found that at low temperatures (below 15 °C) and high NH4 sludge loading (BoTSn above 0,3 kgN/kgoTS*d), the sludge age should be multiplied by a factor of safety ( SF = 2 - 7) to ensure complete nitrification. Biological removal of NH4-N exceeded 95% at a wide pH range (7-9) over a period of two months. Process sludge age was an the average 21 days, BOD and NH4-N sludge loading rates were 0,14 and 0,37 kg /kgoTS*d respectively. Almost 30% increase in nitrifier’s biomass, activity and nitrification efficiency was obtained at pH level below 7. Therefore, to achieve nitrification stability of NH4-rich wastewater, the process pH value should be maintained above 7. The feasibility of single-stage activated sludge to nitrify highly nitrogenous industrial wastewater from steel work plant complex has been verified. An average of NH4-N removal efficiency (90%) could be achieved at COD and NH4-N sludge loading rates of 0,16 and 0,32 kg /kgoTS*d. Average sludge age was maintained at 23 days. However, at higher sludge loadings shock loadings of cyanide might cause drastic increase in NH4-N and NO2effluent concentrations. Pretreatment of the industrial wastewater by alkaline precipitation, activated carbon and additional phosphorus enhanced the nitrification process. This enhancement was accomplished by chemical precipitation of some heavy metals (Zn and Cu) at alkaline pH level, adsorption of cyanide (30%) an activated carbon and providing trace elements (PO4-P) for nitrifiers growth. Difficulties were encountered by BOD determination for industrial effluent samples. This might reflect the nonbiodegradable fraction of the organic matter present in the feed. COD removal efficiency revealed only 49% with a slight positive and linear correlation between COD/N ratio, COD sludge loading, COD removal rates and heterotrophic biomass. Nitrification activity, under various aeration frequency periods and complete anoxic conditions at substrate limiting conditions for a period of almost two months, was investigated. Four reactors (A, B, C and D) were subjected to different aeration frequencies of 30 minutes (A = 7 times/week), (B = 2 times/week), (C = 1 time/week) and reactor D was not aerated. After six weeks operation, the nitrifiers oxygen uptake rates (OURn) reached 14% (reactor A), 29% (reactor B), and 22% in reactor C. The OURn value for reactor D decreased drastically to 10% first after one week. All the NH4 and NO2content in the reactor formed during ammonification was completely nitrified in reactor A, B, and reactor C but not D. Almost 10% decrease in the organic biomass was observed in all reactors due to starvation and decay. The synthetic feed was introduced at the 7th experimental week to all reactors under continuous aeration. Reactor A, B and C retained their nitrifying activity after 5 days operation. However, reactor D always showed NO2 build-up but no NH4accumulation. This suggests that Nitrobacter would have suffered more than Nitrosomonas under anoxic conditions. Further, Nitrosomonas could recover its activity under substrate and dissolved oxygen limitations for a long period. Both under continuous aeration (reactor A) and anoxic conditions (reactor D), the nitrifiers population was subjected to predation and decay. However, under intermittent aeration nitrifiers were able to recover their activity faster than under anoxic one. Therefore, for nitrifying wastewater treatment plants receiving discontinuous flow rate, energy saving might be accomplished by keeping only intermittent aeration. Finally, it should be remembered that these experimental data were obtained from bench scale; Single-stage activated sludge systems treating highly nitrogenous (NH4-N) synthetic and industrial effluents with low carbonaceous (BOD) matter. Any attempt to extrapolate these data on a similar pilot scale, experimental field research program should be carried out to verify comparable results.|
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