A Novel Design Concept of Modified Anaerobic Inclining-Baffled Reactor (MAI-BR) For Wastewater Treatment Application

The successful application of anaerobic technology for the treatment of wastewaters is critically dependent on the use and development of high rate anaerobic reactors. In this study, a novel modified anaerobic inclining-baffled reactor (MAI-BR) which is a combination of regular suspended growth and fixed biofilm systems together with the modification of baffled-reactor configurations was developed. Furthermore, the MAI-BR startup and performance were investigated for the treatment of recycled paper mill effluent (RPME). The start-up has been achieved shortly in 30 days and up to 71% of COD removal was observed, while methane yield increased from 0.018L to 0.808L CH4/day. The performance of MAI-BR treating RPME has showed that COD and BOD removal efficiencies were 94% and 93%, respectively. In the short period of 15 days, the biogas volume increased up to 1.05L per day, while the methane content reached 55%. Finally, a total of 0.55L CH4/day was yielded. These results proved that the design of MAI-BR succeed to treat complex type of wastewater.


Introduction
With the rapid development and escalating crisis of water pollution, water resources are facing increasingly deficient.Therefore, there is a significant need to develop reliable technologies for wastewater treatment.Due to this problem, an anaerobic digestion process for wastewater treatment has gain increasing attention.It has several advantages that have attracted many researchers such as design simplicity, use of non-sophisticated equipment, low excess sludge production, high treatment efficiency and low capital and operating costs [1].Nevertheless, anaerobic baffled bioreactor (ABR) can be described as a series of Upflow Anaerobic Sludge Blanket (UASB) for it can be divided into a few compartments.Figure 1 shows the original design of baffled bioreactor which is vertical in design.Vertical baffles arranged in series forces the wastewater to flow under and over them as it passes from the inlet to the outlet [2].
Bacteria inside the reactor gently rise and settle due to flow characteristics and gas production.The main driving force of an ABR is to enhance the solids retention capacity, however some modification have been made to treat different types of wastewater.Fannin et al. [3] introduced vertical baffle to a plug flow to treat high solids slurry in order to enhance the ability to maintain high populations of slowly growing methanogens.It proved that by applying a vertical baffle, more biomass can be retained so that slowly growing methanogens can actively produce methane gas.From the result, found that methane levels increased from 30% to over 55% with methane yield of about 0.34m 3 /kg VSS.In a later study, Bachmann et al. [4] comparing a baffled reactor before and after narrowing the downflow chambers and slanting the baffle edge, found that production rates and reactor efficiency were improved in the modified design.
The first several hybrid designs were introduced by Tilche and Yang [5], as shown in Fig. 2. The idea to modify the existing design was to enhance solids retention for high strength wastewater treatments.The solid settling chambers were incorporated after the last compartment and the  solids washed out were collected and recycled to the first compartment.Packed Pall rings is randomly positioned at the liquid surface of the first two phase chambers and a deeper structured modular corrugated block which has a high voidage is positioned at the third chamber.Higher loading rates are possible for this structure due to minimal solids washouts during elevated gas mixing.The separation of the gas can enhance reactor stability by shielding entrophic bacteria from elevated levels of hydrogen which was found in the front compartment.
In addition, Boopathy and Sievers [6] further modified the baffled reactor to treat swine wastewater containing high content of small particulate material.The baffled reactor was modified to reduce upflow liquid velocities and to accept the whole waste, as shown in Fig. 2. The first compartment was double in size to 10L followed by 5L of second compartment.It is found that the larger compartment acted as a natural filter and provided superior solids retention for small particles.The reactor collected double the amount of solid material (20.9g/L) than the reactor with three chambers.This was further substantiated in the solid washout data, which was lower in the two-compartment re-actor despite showing lower treatment efficiency.Further analysis showed that despite losing more solids, the threecompartment reactor was more efficient at converting the trapped solids to methane.
The development of effective and simple technologies for wastewater treatment is a challenging task to environmental engineers and scientists.Modification of the basic configuration of an ABR contributes to the development and improved treatment efficiency of wastewater.In this study, a novel modified anaerobic inclining-baffled reactor (MAI-BR) which is a combination of regular suspended growth and fixed biofilm systems together with the modification of baffled-reactor configurations, were developed.Furthermore, the reactor start-up and performance were investigated for the treatment of recycled paper mill effluent (RPME).

Reactor design and fabrication
The schematic diagram of the laboratory-scale MAI-BR set-up is shown in Fig. 3.The MAI-BR is fabricated using polypropyleneplastic, constructed by a dimension of 80cm in length, 15cm in width and 30cm in height (without water jacket), having a total effective volume of 35L (calculated without baffles and packing materials).The MAI-BR consisted of five chambers and each chamber is separated by a modified vertical baffle, the lower portion of the hanging baffles bent to route the flow into the upflow chambers.
Each modified baffle has its own characteristics (form/shape) to facilitate a better contact and greater mixing of wastewater and sludge at lower part of the MAI-BR.About 200g of the packing materials were added to the second and third chamber of the MAI-BR (the detail of baffle modifications and packaging materials are not shown in Fig. 3).The reactor had an attached water jacket to maintain the reactor temperature at 37 • C. Peristaltic pumps are used to control the influent feed rate and effluent recycling to the first chamber of the reactor system.

Substrate and Seed inoculum
Anaerobic sludge from Palm Oil Mill Effluent (POME) (Malpom Palm Industries Bhd, Penang, Malaysia) was collected due to its ease of degradation and high COD value.The sludge was collected and kept in closed containers to avoid biological contamination by air.After collection, the sludge was immediately used and fed to the reactor.On the other hand, recycled paper mill effluent was used as substrate.The wastewater samples were collected from Muda Recycled Paper Mill, Penang, Malaysia and kept in cooling room at 4oC until used in seeding process.

Analytical methods
For sample analysis, triplicate samples were collected for each reading and analyzed twice in order to increase the precision of the results, and only the average value was reported throughout this study.The repeatability of the experimental data was found to be sufficiently high with relative error between repeated runs less than 5%.Sludge, supernatant liquor and gas samples were taken separately for each compartment.Sample analysis including biological oxygen demand (BOD), pH, alkalinity, total solid (TS), suspended solids (SS) and volatile suspended solids (VSS); all according to standard methods [7].
The chemical oxygen demand (COD) was measured using Spectrophotometer DR-2800 according to the reactor digestion method adapted from Jirka and Carter [8].The total volatile fatty acid (TVFA) was determined using Spectrophotometer DR-2800 according to esterification method adapted from Montgomery et al. [9].The microbial floc size was measured using a Malvern Particle Size Analyzer model 2000.The methane gas (CH 4 ) was determined using Shimadzu GC-FID with FTD column.

Reactor operation strategy
In order to investigate the performance of MAI-BR for the treatment of recycled paper mill effluent (RPME), the start-up of the reactor was performed for 30 days using 1000mg/L of COD and HRT of 5 days, giving an organic loading rate of 0.2g COD/L.After the reactor was started, it was operated on continuous mood with 1000mg COD/L and 3 days HRT.An organic loading rate of 0.33g COD/L of RPM wastewater was supplied to the reactor for a period of 15 days.Throughout the operational period, COD, biogas volume, methane content and pH were monitored every two days until steady-state condition achieved.At the steady-state condition, samples from each compartment, influent and effluent were taken and COD, BOD, TSS, VSS, Alkalinity, VFA, pH, lignin and floc size were analyzed.

The modified anaerobic inclining-baffled reactor (MAI-BR)
The lab scale set up of MAI-BR is shown in Fig. 4. The compartmentalization MAI-BR results in a buffering zone between the primary acidification zone (downflow chamber) and active methanogenesis zone (upflow chamber) [1].MAI-BR behaves partly as a fluidized bed reactor similar to up-flow anaerobic sludge blanket (UASB) reactor and it also acts as an activated sludge reactor as well as trickling submerged fixed film reactor.This bioreactor combines suspended-growth and attachedgrowth processes in a single reactor to take advantage of both biomass types which theoretically advantageous to reactor kinetics and process optimization.The most significant advantages of this design was its ability to nearly perfectly realize the staged multi-phase anaerobic theory, allowing different bacterial groups to develop under more favorable conditions, low costs and without the associated control problems.Other advantages include reduced sludge bed expansion, no special gas or sludge separation required and high stability to organic and hydraulic and toxic shock loads.

The start-up of modified anaerobic incliningbaffled reactor (MAI-BR)
Initially, the reactor was started-up on continuous feeding with 1000mg COD/L giving an organic loading rate of 0.2g COD/day of RPME [10].Over 30 days of continuous operation, the system demonstrated a remarkable performance of high COD removal efficiency (up to 71%).Similarly, a maximum COD reduction of 60% was recorded using unmodified anaerobic baffled reactor for the treatment of pulp and paper mill black liquors [11].Nevertheless, the effluent pH was slightly decreased (from 7.3 to 6.2), while the methane production in-creased from 0.018 to 0.808L CH 4 /day in 30 days.It was believed that during anaerobic reactor start-up, the biomass is acclimatized to new environmental conditions, such as substrate, operating strategies, temperature and reactor configuration.Moreover, the methanogens and certain acetogens may be greatly outnumbered by the fast growing acidogens.Consequently, an accumulation of volatile fatty acids (VFAs) and dissolved H 2 were occurred.Therefore, the pH was found slightly decrease since day 1 until day 30.

The performance of modified anaerobic incliningbaffled reactor (MAI-BR)
The performance of the MAI-BR was investigated for the treatment of recycled paper mill effluent (RPME).The continuous operation of the MAI-BR was started using an initial COD concentration of 1000mg/L at HRT of 3 days.For 15 days, MAI-BR was run continuously and observations were made at this particular OLR and HRT.Biogas production, methane composition, pH profile and COD removal efficiency were monitored parameters during the continuous feeding technique.Fig. 5 shows temporal changes in the total RPM COD removal RPME using MAI-BR.
As COD removal rate increased from 81% to 94%, the quick steady state was achieved.The COD removal rate in the reactor was comparatively high which indicate that MAI-BR has successfully performed.The above results on COD removal are comparable with other studies on treatment of municipal wastewater using modified anaerobic baffled reactor, when a total of 84% COD was removed [12].One of the important observations in anaerobic process is pH levels.Fig. 6 shows the pH profile during 15 days of operation.The pH slightly decreased (from 7.1 to 6.7), indicating the adjustment of the microorganisms to maintain the desired pH levels (6.6 to 7.7) for an anaerobic reactor [13].
Methane production during the MAI-BR operation was analyzed and the results are shown in Fig. 7. Through 15 day, the biogas volume was increased from 0.88L to 1.05L per day, while the methane content increased from 28% to 55%.At the end of this phase, a total of 0.54L CH 4 /day was yielded.The increase in methane content may be attributed to the increase in the growth of anaerobes particularly methanogens in the newly synthesized biomass.

The performance of MAI-BR during steady state
The performance of MAI-BR at steady state is shown in Table 1.Eleven parameters were tested for each compartment, influent and effluent.The contribution to total COD and BOD removals by each compartment was digressive longitudinally down the reactor, thus organic matter significantly removed in the 1 st compartment.As illustrated in Table 1, the pH values of each compartment increased in the sequence of flow pattern from the influent to the effluent under all conditions tested.The pH levels in the individual compartments varied in a narrow range with no statistically difference.In addition, a low VFAs accumulation was observed in compartment, whilst the rest compartments maintained the acidity in terms of pH and VFA concentration at favorable levels that allowed effective anaerobic digestion to proceed [14].
sharply in the following compartments was consistent with COD concentration profiles.In term of alkalinity, it was observed the system has enough bicarbonate with slightly constant value of 214mg/L throughout all compartments.It's believed that effluent recycling has enhanced the alkalinity content in the system, system, was also found to alleviate the problems of low pH.Nevertheless, neutral pH level, high alkalinity and low VFAs suggested that methanogenesis appears to be dominant in all compartments.
Table 1 shows the concentrations of TS, TSS and VSS for each compartment influent and effluent.The influent solids (TS) concentration is 1382mg/L with 83% of it as total suspended solids (TSS), whilst the total suspended solids (TSS) concentration is 1142mg/L with 89% of it as a volatile fraction (VSS).Up to 73%, 83% and 89% removals were observed for TS, TSS and VSS, respectively.The TS, TSS and VSS concentrations in the treated effluent were 368mg/L, 192mg/L and 116mg/L, respectively.A floc samples were taken from the bottom of each compartment and analyzed for its size.Compartments 1 and 2 had the largest floc size as it's contain the highest COD levels which encourage the growth of microbes in the front of the reactor.The floc size decreased down of the reactor due to the low substrate.Lignin content in influent and effluent were 13.75mg/L and 6.5mg/L, respectively.About 53% of the lignin was removed even though there is no clear understanding on the biodegradability of lignin in anaerobic system [15].

Conclusion
The design of MAI-BR shows that it is capable to treat complex type of wastewater.The physical structure of the reactors allows various modifications to be made that providing the capability to treat wastewaters that currently require at least two separate units, therefore substantially reducing capital costs.During 30 days of the reactor start-up, up to 71% of COD removal was observed, while methane yield increased from 0.018 to 0.808L CH 4 /day.Even though slightly low pH levels were noted, high COD removal efficiencies confirm the ability of MABR configuration to overcome the adverse effect of pH.The performance of MAI-BR treating RPME has showed that COD and BOD removal efficiencies were 94% and 93%, respectively.A total of 73%, 83% and 89% removals were observed for TS, TSS and VSS.In the short period of 15 days, the biogas volume increased up to 1.05L per day, while the methane content reached 55%.Finally, a total of 0.55L CH 4 /day was yielded.These results clearly indicate that the start-up and operation of MAI-BR led to a more complete biological degradation of the organic matter and a better adaptation of the biomass for the degradation of the substrates.

Figure 1 :
Figure 1: Original design of vertical baffled reactor.

Figure 2 :
Figure 2: Modifications of the anaerobic baffled reactor.

Figure 5 :
Figure 5: COD removal efficiency profile (%) at OLR of 0.33 g COD/day and HRT of 3 days.

Figure 6 :
Figure 6: Effluent pH profile at OLR of 0.33 g COD/day and HRT of 3 days.

Figure 7 :
Figure 7: Methane production volume profile (L) at OLR of 0.33 g COD/day and HRT of 3 days.

Table 1 :
The performance of MAI-BR at the steady state condition.