A novel environmental biotechnological aerobic process (BioAX) for the treatment of coal gasification wastewater

Abstract

A high efficiency advanced bio-membrane technology aerobic reactor (BioAX) has been investigated for the treatment of coal gasification wastewater over a period 300 days. It could treat the coal gasification wastewater effectively after a post anaerobic process. With the influent conc. of COD 800–900 mg L⁻¹, total phenol 100 mg L⁻¹, ammonium nitrogen 80–100 mg L⁻¹, and volatile phenol 40 mg L⁻¹, the effluent COD, total phenol, ammonium nitrogen, and volatile phenol could decrease to 200–300 mg L⁻¹, 20 mg L⁻¹, 15–30 mg L⁻¹ and 1 mg L⁻¹ showing average removal efficiencies of COD, total phenol, ammonium nitrogen, and volatile phenol of 70–80%, 80%, 70–80%, and 99.9%, respectively. The loading rate and removal rate of total phenol showed a linear relationship having R² = 0.95169. The technology also substantially reduces the sludge quantity and thus reduces the sludge handling cost. After 300 days of continuous operation, scanning electron microscopy (SEM) revealed the formation of biofilm by the microbial population as well as intensive filamentous bacteria settlements on the biological filler. Moreover it has a durable aeration system that is easy maintained and can be easily replaced. It can treat low to medium contents of organic or biological waste.

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1. Introduction

Coal gasification plays a critical role in overcoming the limited supply of natural gas in China. During its manufacturing processes a coal gasification plant consumes a large amount of water. The wastewater is discharged mainly from the gas washing and condensing operation of the coal gasifier.¹ However, the coal gasification wastewater (CGWW) contains complicated contaminants and high concentrations of toxic compounds, like phenol, cyanide, thiocyanate and ammonium.^2–4 The phenolic components, primarily phenol, methyl phenol, C₂- phenol constitute over 60 to 80% of the total organic content of the CGWW.^5–7 It also includes typical refractory organics such as polynuclear aromatic hydrocarbons (PAHs) and nitrogenous heterocyclic compounds, most of them have been reported to be carcinogenic and mutative.^8–14

CGWW is commonly treated by the dual process of physic-chemical pre-treatment and biological process.^15,16 Although, this dual process are very effective, but still confront with some issues, like complicated technology, large area is occupied, along with secondary pollution production such as sludge in the biological process.¹⁷

The anaerobic process provides cost-effective and efficient techniques for the treatment of coal gasification wastewater, but reducing effluent COD to a level below 200 mg L⁻¹ remains difficult^18–20 and also the absolute degradation of the volatile phenol, total phenol and ammonia nitrogen is quite difficult.

In 1990s, the use of biofilm system for aerobic system has been promoted to overcome the drawbacks connected with activated sludge system. Biofilms were more efficient for wastewater treatment than suspended activated sludge it has been proved by the intensive research in the field of biological wastewater treatment during the last 20 years.²¹ A biofilm is any group of microorganisms in which cells stick to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS) but the main drawbacks of this system are the relatively high investment costs associated to the use of support materials for biomass attachments.²² In order to get the advantages of biofilm system, without the use of carrier materials, in recent year's researches shows that it is possible to grow aerobic granular sludge (AGS) in either continuous or batch-wise operated system or sequencing batch reactor.^23–29 The aerobic granular sludge technology is a special kind of biofilm structure composing of self-immobilized cells.^21,30

The practical application of AGS technology has been hindered due to the loss of its microbial activity under extended ideal conditions or after long term storage, the cultivation of AGS, has restricted the development of AGS technology from lab scale to pilot scale.^18–20

In order to overcome these weaknesses of AGS, researches have been devoted to the development of a better treatment technology. The development of advanced bio-contact (attached growth) aerobic process BioAX with internal circulation is done by our research group it is a further step as an advanced aerobic system, similar to the attached growth process, which is faster and more efficient than the conventional activated sludge process. It has a significantly higher volumetric organic loading capacity compared to the conventional suspended growth process. Advantages over conventional bio-contact oxidation process can be seen in Table 1.

Table 1 Comparison with conventional bio-contact oxidation process

	Conventional	BioAX
Growth of biofilm	Intensively disturbed by aeration	Not disturbed by aeration
Flow pattern	Completely mixed	Plug flow with overturning
Air distribution	Non uniform	Uniform
Shortcut flow	Exist	Not exist
Oxygen transfer efficiency	Higher aeration rate needed	Higher oxygen transfer efficiency
Biological chain	Simple	Running in sections with dominant bacteria respectively
Effluent quality	Turbid	Good
Maintenance	Frequent wash needed	Negligible

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It added an internal recycle in the bio-contact oxidation i.e. an improvement in original bio contact oxidation process. The structure of reactor and flow path enhances the circulation of the fluid in the reactor after adding internal circulation tube, wastewater was pumped into the bottom of the reactor; with an increase of air flow in this area (air ratio is also high) the gas–liquid transfer process mainly takes place here. When the gas–liquid goes to the top of the reactor, the gas–liquid separation occurs. Most of the gas move out of the liquid and the wastewater moves to two sides of the reactor due to gravity and dam-board, it flows downward along the downstream area (this wastewater contains less gas or even no gas). When it goes to up flow zone, after it reach the bottom of the reactor together with influent and gas it reaches to the aeration mouth, circulation process is generated (the gas is denser in inner side in comparison to outside) due to the density difference the circulation will be more easy. This process is better than external circulation loop style. The internal recycle has a compact structure; it can be divided into multistage to enhance the cycling speed. The water in the BioAX experiences back-and-forth flow, filtration occurs in each carrier layer every time the water flows downward, thus forming a process of multilevel filtration.

The aim of this work was to assess the technical feasibility of using a BioAX in the aerobic treatment of coal gasification wastewater post anaerobic process³¹ by setting-up process conditions including the adaptability of the system to sudden variations in organic load. Finally, the microbial population developed on the surface of the biofiller were also characterized.

2. Materials and methods

2.1 Pilot scale aerobic reactor

BioAX designed is based on the principle of central tube airlift reactor (Plexiglas, effective volume 8.0 L; inside diameter 0.15 m; height 1 m; operating temperature 15–20 °C) Fig. 1. Plastic packing containing microbial species which is used for the aerobic attached growth process is installed at aeration tank; the direction of the flow water is downward. The bio-filler (brush-like), stuffing gap is so big there is need to back washing and their distance is optimize in order to ensure that blockage does not occur, so no need for reverse flushing as shown in Fig. 1. The aeration system ensures proper flow of wastewater in the aeration tank without short-circuiting.

Fig. 1 Aerobic reactor apparatus along with schematic diagram of biological filler.

As can be seen in the Fig. 1 as the air flow flows from bottom to top in the central tube, it elevates water from bottom of the central tube to the top of the BioAX. The water flows up and again it flows downward outside the central tube along the carriers. So the flow of water was continuous in a loop manner under the effect of gravity. The solid–liquid separation was conducted by the flow of water passing through the carrier layer as SS was filtered by the carriers and gets separated finally.

In practical engineering applications, the reactor has horizontal distribution of circulation pipes, so the flow of water is horizontal from the inlet port to the outlet port but in the case of BioAX the circulation pipes are vertical so, the water flow can be horizontal as well as vertical, water flow is evenly and it has a good contact with biofilm.

2.2 Aerobic batch experiment

Batch apparatus with effective volume of 6 L was mounted with aeration head connected to draught fan. The reactor was equipped with jacket layer connected with circulating water to heat the reactor and to regulate the temperature (about 20 °C). The inoculated sludge was 1 : 1 (MLSS = 5000 mg L⁻¹) mixture of two kinds of sludge: one was aerobic sludge domesticated in our lab, and the other sludge was from the SBR reactor from a sewage treatment plant in Suzhou, China. Influent was taken from anaerobic effluent of coal gasification wastewater treatment process. Sampling time was 5, 18, 30, 42, 54, 66, 78, 90 hours. COD and total phenol were analysed.

2.3 Operating conditions and method

Coal gasification wastewater composition is shown in Table 2. The test water entering in aerobic reactor was taken from the prestart stage of anaerobic reactor. The pH value of anaerobic effluent was adjusted to pH 8 with sodium bicarbonate before entering the aerobic reactor. The sludge was taken from the sewage wastewater treatment plant in Suzhou, China and was acclimatized for 140 days.

Table 2 Characteristics of the raw coal gasification wastewater^a

Parameter	Value
a unit: mg L^−1, except pH.
COD	3800–4400
BOD	500–700
Total phenols	850–950
Volatile phenols	450–530
Total nitrogen	240–320
Ammonia nitrogen	230–300
Volatile acids	80–120
Total phosphorus	0.238
Suspended solids	300–400
Oil	20–30
pH	8.5–9

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2.4 Analytical methods

Ammonium nitrogen was measured by a colorimetric method.³² It was based on the reaction of NH₃ with HClO and phenol, forming a strong-blue compound (indophenol) which can be colorimetrically determined using a spectrophotometer (Shimadzu UV-1603, UV-visible) at 635 nm. The pH is one of the key parameters measured in wastewater treatment systems, since its control is important to maintain the biological activity of the microorganisms involved in the treatment process. The pH measurements were performed with an electrode (Crison Instruments, S.A., 52-03) equipped with an automatic compensatory temperature device (Crison Instruments, S.A., 21-910-01) and connected to a measure instrument (pH mV⁻¹). COD, volatile phenol, total phenol analyses were carried out by the standard methods for the examination of water and wastewater.³³

Microorganism in the biomass were observed using a scan electron microscope (Digital SEM Leica 440 at 20 kV) controlled with a computer system.

3. Result and discussion

3.1 Aerobic batch test result

The aerobic batch test results are shown in Fig. 2 and Table 3. COD degradation takes place efficiently from 793 mg L⁻¹ to 397 mg L⁻¹. COD removal efficiency reached 50% in the beginning of 30 h of the batch experiment. COD removal degradation slows down with increase in reaction time (78 h), COD reduced to 356 mg L⁻¹, with removal efficiency of 55%. During 48 h, 41 mg L⁻¹ of COD was removed; its removal efficiency only increased by 5%. The trend of total phenol degradation in first 30 h was faster. Total phenol conc. decreased from 164 mg L⁻¹ to 50 mg L⁻¹ with removal efficiency of 70% but with subsequent increase of reaction time the TP shows an upward trend. So degradation of organic compound by aerobic microorganism in wastewater was done by 2 process i.e. adsorption and degradation. The TP conc. increase after 30 h was mainly due to the release of TP concentration by microorganism. According to the intermittent results of aerobic batch experiment HRT of 30 h was best as when reaction time (equivalent to HRT) was over 30 h, the change of COD concentration was quite small. So, the aerobic batch HRT of 30 h was best can be seen from Fig. 2.

Fig. 2 Aerobic intermittent test (a) COD, total phenol variation trends (b) COD and total phenol removal.

Table 3 Aerobic batch test results

Reaction time (h)	COD (mg L^−1)	Total phenol (mg L^−1)	COD removal efficiency %	Total phenol removal efficiency %
0	793	164.3	0	0
5	593	130.7	25.2	20.5
18	442	73.2	44.3	55.4
30	397	49.5	49.9	69.9
42	372	51.5	53.1	68.7
54	421	53.4	46.9	67.5
66	364	59	54.1	64.1
78	356	73.2	55.1	55.4
90	368	87	53.6	47.0

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3.2 Operational performance of BioAX reactor

Aerobic reactor run for 302 days, the performance of aerobic reactor was affected by the anaerobic reactor effluent quality, when the anaerobic reactor run in the stable operation phase, the aerobic reactor showed a good and stable treatment effect.

In Fig. 3(a) and Table 4 at initial stage the sludge content was relatively high in the system; therefore removal efficiency of COD was relatively higher (around 70%). In the following 13–30 days, effluent COD of aerobic reactor gradually increased when COD removal dropped to about 50%, indicating that aerobic microorganisms had inadaptability in the environment of SNG anaerobic effluent (short period of about 17 days). From 31–60 days, aerobic microorganisms gradually adapt to the new environment. The effluent COD decrease to 300 mg L⁻¹ with removal efficiency of 80%. Although, the effluent quality of anaerobic reactor shows fluctuations between 1000–1400 mg L⁻¹ in the start-up phase, but aerobic reactor showed fairly stable removal efficiency. This shows that biodegradability had been enhanced after hydrolysis acidification stage of SNG wastewater, its increases, so aerobic microorganisms can adapt quickly in SNG wastewater and exhibit relatively high activity by a short acclimation period.

Fig. 3 Aerobic reactor degradation trends.

Table 4 Aerobic reactor test water quality and operating conditions

Phase	Operating time (days)	HRT (h)	COD (mg L^−1)	Total phenol (mg L^−1)	Volatile phenol (mg L^−1)
1	1–88	125	800–1380	200–260	20–120
2	89–117	94	720–990	210–270	40–120
3	118–200	63	430–920	150–270	20–130
4	201–302	64	370–600	100–120	10–30

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In the second and third operational phase, COD removal of aerobic reactor maintained at 70–80% with the effluent COD conc. stabilized at about 200 mg L⁻¹ aerobic reactor stopped running after a period of time, we restart the reactor by adding some activated sludge from the wastewater treatment plant. It can be seen from Fig. 3(a), COD removal efficiency was 30% after the initial restart stage. The removal efficiency recovered to 60% after 10 days of acclimation, and it shows stability. Since influent COD was around 500 mg L⁻¹; the aerobic effluent COD maintained at 200 mg L⁻¹. So, in the aerobic phase COD removal efficiency was about 60%. At HRT 5.2 days (1–88 days) the organic loading rate was (max 292.99 mg COD per L per day and mini 120.58 mg COD per L per day), HRT 3.9 days (89–117 days) the organic loading rate was (max 246.89 mg COD per L per day and mini 185.11 mg COD per L per day), HRT 2.6 days (118–200 days) the organic loading rate was (max 447.62 mg COD per L per day and mini 152.38 mg COD per L per day), HRT 2.7 days (201–302 days) the organic loading rate was (max 238.13 mg COD per L per day and mini 151.13 mg COD per L per day).

From Fig. 3(b) it can be concluded that removal of phenolic compounds by aerobic microorganisms was superb, basically maintained at above 80%; volatile phenols (VP) was undetectable in aerobic effluent, so VP removal efficiency was 100%. Total phenol and COD removal exhibit similar situation in aerobic reactor. During the operating time of the reactor from 18–54 days, the TP concentration in the effluent shows upward trend from 50 mg L⁻¹ to 133 mg L⁻¹ this was due to loss of sludge in aerobic reactor.

At 55^th day, activated sludge was added in the reactor to reach 5000 mg L⁻¹, the total phenol removal efficiency immediately shows a increased trend, at 64^th day TP conc. was only 30 mg L⁻¹. During subsequent operation TP concentration was maintained from 27–30 mg L⁻¹, with removal efficiency of 80%. In the first 200 days after the re-start of the aerobic reactor, microorganism capable of phenol degradation ability needs about 20 days to restore their phenol degrading tendency. During stable running of aerobic reactor TP conc. was approximately 20 mg L⁻¹, the removal efficiency was about 80%.

Fig. 4(a) illustrates the relationship between HRT, TP loading rate and TP influent, the loading rate shows consistency with the HRT only a small change was observed while COD influent was changing. Starting from day 1–88 with HRT 5.2 days, loading rate (max. 55.68 mg per L per day and min. 22.66 mg per L per day); for HRT 3.9 days from 89–117 days, loading rate (max. 69.96 mg per L per day and min. 48.00 mg L per day); when the HRT was further lowered to 2.6 days from 118–200 days, loading rate (max 111.24 mg per L per day and mini 51.43 mg per L per day); and for HRT 2.7 days from 201–302 days, the loading rate (max 86.7 mg per L per day and mini 31.50 mg per L per day). Fig. 4(b) shows that the loading rate and removal rate of total phenol has a linear relationship (R² = 0.95169).

Fig. 4 Relationship between (a) HRT, TP loading rate and TP influent (b) TP removal rate and loading rate.

The influent and effluent ammoniacal nitrogen (NH₃–N) concentration was irregular during the stable running of secondary start of aerobic reactor. Results in Table 5 shows that influent NH₃–N concentration was about 80–100 mg L⁻¹, effluent NH₃–N concentration was about 15–30 mg L⁻¹, NH₃–N removal efficiency was 70–80%.

Table 5 Ammoniacal nitrogen (NH₃–N) concentration during the running of aerobic reactor

Operating time (days)	Influent ammoniacal nitrogen (mg L^−1)	Effluent ammoniacal nitrogen (mg L^−1)	Ammoniacal nitrogen removal efficiency (%)
234	83.4	22.7	72.8
246	96.8	28.7	70.4
255	91.0	26.4	71.0
269	76.6	17.6	77.0
282	94.3	27.8	70.5
291	78.7	14.3	81.8

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3.3 Characterization of the bio-contact (attached growth)

Fig. 5 shows the Scanning Electron Microscopy (SEM) of the aerobic bacteria on the surface of the bio-filler. Bacteria can easily be seen attached to the biofiller. The specially designed plastic packing which contains high concentration of high-efficiency microbial species which are capable of degrading organic compounds such as PVA, they are used as the aerobic attached growth process to enhance removal of organic matters in wastewater as shown in Fig. 5(a) biofilm growing on the surface of the biofiller can be seen clearly in the SEM images, Fig. 5(b) shows the formation of biofilm by filamentous bacteria, Fig. 5(c) showed lush breeding colonies, intensive filamentous bacteria settlements on biological filler, Fig. 5(d) filamentous rod shaped microorganism bacillus and cocci were present predominantly and Fig. 5(e) biofilm formed by rod-shaped bacteria.

Fig. 5 (a) Biofilm on the surface of filler (lush breeding microbial colonies) (b) and (c) SEM of biofilm (d) and (e) colonies of cocci and rod shape filamentous microorganism.

3.4 The structural characteristics of aerobic sludge microorganisms

Fig. 6(a–d) shows micrograph of sludge in the aerobic reactor. There are quite a few protozoa's and metazoans in the aerobic reactor. Fig. 6(e) and (f) shows SEM of sludge in the aerobic reactor after running for 128 days. There are some filamentous bacteria and remnants of protozoa's.

Fig. 6 (a)–(d) Micrograph of sludge in the aerobic reactor (40 times), (e) & (f) SEM of sludge in the aerobic reactor.

3.5 Reduce biomass production & cost effective method

This process shows the reduction in the biomass production as suspended solids in the water is filtered by the carriers and gets intercepted there, in turn becoming prey to the microorganism growing on the surface of the biofilm as well as the microbial phase in the biofilm is abundant it varies from bacteria to protozoan and metazoan, and thus forming a food chain. The predation leads to the reduction in sludge production.

BioAX technology also significantly reduces the land area occupied by the wastewater treatment plant. It consumes less electricity as the aeration system is more effective and lower blower capacity, very durable (easily replaceable), capacity for volumetric organic load is higher, no need for replenishment of microorganism, shorter microorganism cultivation time so start-up is faster. It is technically feasible as well as a cost effective method in terms of lower power consumption. In the oxygenating mode (the renewal of gas–liquid interface in the process of gas–liquid turbulence) the resistance of air flow was small, and the aerator was macro porous, so the oxygen transfer efficiency was 20–30% higher than micro porous aeration. The air flow which was required was small, so the power consumption was low.

Power consumption in aeration = wind pressure (P) × air quantity (Q).

If water flow rate is 150 m³ h⁻¹, total phenols concentration 120 mg L⁻¹, COD 800 mg L⁻¹, ammonia nitrogen 310 mg L⁻¹, then the roots blower selection is C308A/B, flow 87.3 m³ min⁻¹, pressure 49 kPa, power 110 kW, motor speed 800 rpm, 2 units. Then the electricity cost: 0.10 $ per kW h × 2673.2 kW h per day = 267.32 $ per day.

Treatment cost per ton of water: 0.07 $ per ton of water.

4. Conclusion

In the aerobic treatment unit, the pre-run was of 150 days, the stable running was more than 300 days. We achieve to these conclusion.

• Acc. To intermittent test result the hydraulic retention time was 30 h.

• It effectively removes organic contents (middle to high COD values) in wastewater exhibiting higher treatment efficiency as the aerobic influent COD 800–900 mg L⁻¹, effluent COD 200–300 mg L⁻¹, the removal efficiency 70–80%.

• When the TP conc. in aerobic influent was 100 mg L⁻¹, the effluent TP conc. 20 mg L⁻¹ or less; TP removal efficiency of aerobic reactor was 80%.

• The volatile phenol was not detectable after the treatment, NH₃–N removal efficiency 70–80%.

• It exhibit stable operation, easy maintenance, unlike conventional activated sludge process, BioAX does not require returned sludge. This has significantly simplified the operation process of the aerobic treatment plant. The technology also substantially reduces the sludge quantity and thus reduces the sludge handling cost. The BioAX is further employed by our research group for the key depth technology research for the treatment of coal gasification wastewater.

• It's an efficient sewage treatment post anaerobic process. It's nevertheless to say that BioAX is the world's first high efficiency bio-membrane technology complete device which allows treated water to reach the requirements for reuse water.

Acknowledgements

The authors express their gratitude to the School of Environmental Science and Engineering, Shanghai Jiao Tong University for providing the research facilities.

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