The most important adverse environmental impacts associated with improper discharge of municipal wastewater having significant amounts of organic matter (COD), nitrogen (N) and phosphorous (P) include promotion of Eutrophication, toxicity to aquatic organisms and depletion of dissolved oxygen receiving streams[1,2]. Due to these adverse impacts, completed treatment of municipal wastewater before discharge has been increasingly needed.
Currently, the most of municipal wastewaters are treated in an activated sludge process to meet the standards for direct discharge. Although, the new discharge standards for nitrogen and phosphorous will lead to the upgrading of the existing wastewater treatment plants and to the conception of new intensive advanced technologies. In recent years there have been many researchers considering the complete treatment of wastewater but they have all more or less dealt with different combination of biological process in separated reactors. Biofilm processes in particular submerged fixed bed reactors developed during recent years are a good alternative for improved wastewater treatment.
Therefore, the aim of present investigation was to design and construct an Up-flow Anaerobic/Aerobic Fixed Bed (UA/AFB) combined reactor in which an anaerobic and aerobic zones could run in a single reactor to study simultaneously carbon and nutrient removal.
MATERIALS AND METHODS
The experimental set up: The UA/AFB reactor as showed in Fig.
1, was made from plexiglas plate consisting of two main anaerobic and aerobic
parts separated with a distributing perforated plate. The reactor was filled
with corrugated plastic tubes as a media. Sampling ports were provided in each
part for sample collection. Table 1 lists the dimensions of
the investigated UA/AFB reactor.
Operating conditions: Synthetic wastewater was prepared using glucose as a source of carbon, ammonium chloride as a nitrogen source and sodium nitrate as a nitrate source, to investigate denitrification in the reactor, in concentrations which were close to those found in municipal wastewaters. The average characteristics of synthetic wastewater are shown in Table 2. Reactor was operated in an up-flow mode.
To develop biofilm on the media, reactor was inoculated with 2 L of thickened
activated sludge from a local wastewater treatment plant.
|| Dimensions of the studied UA/AFB combined reactor
|| Characteristics of synthetic wastewater
|*All parameters in mg L-1 except pH
Schematic figure of UA/AFB experimental combined reactor
The biofilm development
was achieved using a medium 3 times as concentrated as the characteristics listed
in Table 2 and running the reactor first for 10 days in a
batch mode and then over to the continuous mode for 5 days at Hydraulic Retention
Time (HRT) of 24 h and a 2 times concentration of the synthetic wastewater.
After biofilm formation, reactor was run at different HRTs based on wastewater flow rate change with the stable COD, N and P concentration in influent. At any set of the experiment HRT, reactor was monitored to achieve the steady state condition in effluent. When the effluent COD from any parts of reactor did not vary significantly for five consecutive samples, a steady state condition in COD removal was assumed to have been achieved.
Sampling and analysis: Grab samples were collected from influent and sampling ports showed in Fig. 1. Samples were analyzed for COD (5220 C. Titrimetric method), total ammonia nitrogen (4500-NH3 A. Titrimetric method), nitrate nitrogen (4500-NO3¯ B. UV Spectrophotometric screening) and total phosphorous (4500-P C. Vanadomolybdophosphoric Acid colorimetric method) according to standard methods. All parameters were measured on alternate days except for pH, DO and alkalinity witch were monitored daily. DO was analyzed by an YSI DO meter, pH by a pH meter and alkalinity according to standard methods.
The reactor performance for nitrogen removal was evaluated by the apparent volumetric of nitrification and denitrification rate (RN, g N m-3/day) in each part of reactor:
Where, S0 and S are the initial and effluent substrate concentration as mg L-1 calculated based on overall bed volume.
RESULTS AND DISCUSSION
COD removal: Figure 2 shows variation of the effluent COD and COD removal efficiency to achieve the steady sate condition in operation during start-up. As indicated, after passing 17 days from starting up, reactor efficiency in COD removal was stable in 94%. It present that the suitable biofilm have been formed.
Table 3 indicates average values of COD removal efficiency at different organic loading rate corresponding to HRTs of 5 to 24 h.
Figure 3 gives the overall and separated COD removal in the
reactor at different HRTs.
||Profile of COD removal and effluent concentration during start-up
|| Reactor performance in COD removal at different HRTs in steady
|| Performance of the reactor in ammonia nitrogen removal at
COD reduction profile in overall and different parts of reactor
According to the Fig. 3, most of
COD removal occurred in the aerobic part of the reactor. This was expected, because aerobic bacteria are dominate in this part having
higher organic biodegradation rate. As HRT was increased in range of 5 to 24
h, the overall efficiency of COD removal increased. It seems that HRT of 5 h
in which COD reduced to 94%, could meet discharge requirements in Iran. According
to Iran Environmental Protection Agency standards, monthly average of effluent
COD to discharge into surface water is 60 mg L-1.
The relationship between COD loading rate and COD removal efficiency was linear with R2 of 0.97. Westerman et al. similarly obtained a linear relationship between COD loading rate and its removal in an up-flow aerated filter.
Nitrification: Ammonia nitrogen removal (Nitrification) was studied
simultaneously with COD removal in the developed reactor. Table
4 shows NH3 loading and nitrification efficiency data in overall,
anaerobic and aerobic parts of the reactor. Figure 5 shows
these variations graphically. As indicated in Fig. 5 and Table
3, percent of ammonia nitrogen removal in the anaerobic is very low, in
which the removed ammonia have been consumed for cell synthesis.
||Profile of nitrification efficiency variations versus HRT
In the other word, nitrification only occurred in aerobic zone of the reactor.
Table 4 shows that as the HRT have been increased in range
of 5 to 24 h, nitrification increased from 86 to 96%.
Figure 4 indicates that HRT of 7 h is optimum for sufficient nitrification in the tested reactor in witch nitrification efficiency was 94%. In this HRT, ratio of COD removal rate to nitrification rate was close to 10. According to Ros et al., the higher concentration of organic matter in the lower HRT lead to a competition for oxygen in biofilm between heterotrophic (COD removal) and autotrophic (nitrifiers) organism.
In Fig. 4 the nitrification efficiency have been plotted versus HRTs of between 5 to 24 h. It can be seen that while the ammonia concentration was consistent at all HRTs, nitrification efficiency varied at different HRT. A possible explanation of this situation is the difference in COD removal efficiency of anaerobic zone of the reactor and in turns, organic loading on aerobic zone in where nitrification occurred. As HRT was increased, COD removal efficiency in anaerobic zone of the reactor increased and COD loading on aerobic zone decreased that was resulted in increasing nitrification efficiency.
Figure 5 shows the profile of nitrification rate variations
versus ammonia nitrogen loading rate.
||Reactor performance in nitrate removal at different HRTs in
steady state operation
||Reactor performance in phosphorus removal at different HRTs
in steady state operation
||Relation between ammonia loading and nitrification rate
As indicated, nitrification rate have
increased with increasing ammonia loading varying between 148.8 to 34.6 g NH3-N
m-3/day. The results suggest that the nitrification may was inhibited by substrate
(ammonia) concentration so that increasing of the ammonia loading have leaded
to increasing the nitrification rate.
Denitrification: To study the ability of the reactor in removal of ammonia nitrogen (Denitrification), wastewater having nitrate concentration of 24 mg L-1 was pumped into the anaerobic part of the system. The COD to NO3-N ratio in the influent wastewater was around 15. Table 5 shows the denitrification efficiency in overall and anaerobic part of reactor at different HRT. It should be emphasis that denitrification was not occurred in aerobic zone that is according to denitrification theory.
As indicated in Fig. 6, increasing HRT was resulted in decreasing
denitrification efficiency. Maximum denitrification efficiency of 94% obtained
in HRT of 5 h and minimum of that was 91% in HRT more than 15 h.
|| Variation of denitrification efficiency versus HRT
||Relation between nitrate loading and denitrification rate
The higher denitrification efficiency obtained in lower can be explained due
to the higher anaerobic biodegradation products favorable for denitrifirs such
as acetate in the lower anaerobic HRT that is the most efficient substrate for
denitrification. Ros et al. showed that acetic acid which
is transformed to acetate, is dominating product of acidification in low HRT
in the anaerobic degradation of organics.
Figure 7 indicates a linear relation between NO3-N loading and denitrification rate with R2 0.97.
Phosphorus removal: The results obtained from the tested reactor for
phosphorus removal have been shown in Table 6. The overall
phosphorus removal efficiency was low and maximum removal percent occurred in
HRT of 24 h. The results indicate that phosphorus removal is related to cell
synthesis and as expected, the reactor did not show acceptable efficiency from
wastewater. Increased phosphorus removal could be obtained by chemical precipitation
or possibly by adding a biological suspended growth system to promote biological
luxury uptake of phosphorus as described by Goncalves et al..
The results of the examination of a new Up-flow Anaerobic/Aerobic Fixed Bed
reactor can be summarized as follow:
||The tested UA/AFB reactor is a compacted system and achieves
high COD removal ranging from 94 to 97.7% corresponding to HRTs of 5 to
24 h, from municipal wastewater.
||This reactor was able to remove more than 94% of ammonia nitrogen from
influent wastewater in HRT of 7 h which is highly preferable to conventional
biological nutrient removal.
||As expected, the reactor did not show good performance in phosphorus removal
so, to achieve more efficiency combining a biological or chemical process
||In overall, the results obtained showed that the HRT of 7 h is optimal
for simultaneous COD removal, nitrification and denitrification. In this
HRT efficiencies are 95.4, 94 and 94.5% for COD removal, nitrification and