Rapid population growth, industrial development and stringent demand for treatment of wastewater require developing and emerging economies to upgrade existing wastewater treatment plants (WWTPs) or planning new WWTPs. In the context of unavailability or unaffordability of land and resources for infrastructure expansion, low cost, small footprint, less energy consumption and product reuse are some of the major factors to be considered when either upgrading or designing new WWTPs in developing and emerging economies. Although the transition from activated sludge to biofilm processes has partly solved these challenges, there are innovations that can make the processes even more compact and more efficient. Newly developed CFIC (continuous flow intermittent cleaning) process is the next generation moving bed biological wastewater treatment system and is an example for addressing these issues. The CFIC pilot studies showed promising performance for biological chemical oxygen demand and nitrogen removal as well as particle separation facilitating wastewater reuse.

  • CFIC (continuous flow intermittent cleaning) process is the next generation of the well-known moving bed biofilm reactor (MBBR) process.

  • CFIC process was studied for nitrogen removal by nitrification and denitrification.

  • Bio-solids separation was studied.

  • Oxygen transfer efficiency of CFIC and MBBR was compared by off-gas testing.

Graphical Abstract

Graphical Abstract
Graphical Abstract

The activated sludge (AS) treatment process is one of the most popular biological treatment processes of wastewater in the majority of the developing and emerging economies. Land availability, less complexity and less skilled labour demand are some of the reasons for the AS being popular (von Sperling 2007). Rapid population and industrial growth and stringent effluent demand are alarming for both upgrading the existing wastewater treatment plants (WWTPs) and designing new WWTPs in the region. Low cost, small footprint, less energy consumption and product reuse are some of the major factors to be considered when either upgrading an existing WWTP or designing a new WWTP in the developing countries.

Biofilm-based processes are efficient alternatives addressing these factors compared to the conventional AS process (Odegaard 2006). Among different biofilm-based treatment technologies the moving bed biofilm reactor (MBBR), developed in Norway in the late 1980s and early 1990s (Odegaard et al. 1994; Odegaard et al. 1999) is the most popular biofilm-based process due to its advantages like effectiveness and small footprint (Odegaard 2006). The continuous flow intermittent cleaning (CFIC) process was introduced a few years ago as the next generation of the MBBR process (Rusten et al. 2011). CFIC further reduces the footprint and energy consumption by facilitating a compact and efficient treatment process.

The CFIC biofilm reactor concept and process have been explained in detail by Rusten et al. (2011) and Stang et al. (2013). The CFIC reactor typically has a higher biofilm carrier filling fraction than the MBBR, from 80% up to 90–99% of the bulk volumetric fill in aerobic reactors. Due to the specific gravity (between 0.94 and 0.96) of the biofilm carrier material, the carriers are slightly buoyant and packed at the upper part of the reactor during the normal mode with calm upwards flow. The higher filling fraction increases the specific protected surface area (m2/m3 reactor volume) for biofilm growth resulting in a small footprint. The packed carrier bed facilitates better air distribution and good contact between water, substrates and the biofilm grown on the carrier surface. The compacted carrier bed also functions as a ‘filter’ for the particles that come with influent water as well as the bio-solids produced in the reactor. Intermittent reactor washing removes the retained particles and the bio-solids, facilitating separate bio-sludge treatment (Rathnaweera et al. 2016).

This paper presents the results of a complete CFIC pilot followed by membrane separation (CFIC-Mem). The major focus of this study was to evaluate the CFIC process for nitrogen removal. To evaluate the applicability two wastewater qualities, screened and de-gritted wastewater (SDWW) and primary treated wastewater (PTWW), were studied with different influent flow rates, DO concentrations and recirculation. To study the appropriateness of a compact tertiary treatment, coagulation and membrane filtration for phosphorus removal from the CFIC effluent was evaluated. Finally, the oxygen transfer efficiency of CFIC was measured and compared to MBBR by off-gas testing.

CFIC pilot testing was carried out at the Nedre Romerike (NRA) WWTP in Lillestrøm, Norway. NRA is one of Norway's largest WWTPs, having a full-scale MBBR process for nitrogen removal. The full-scale treatment process consisted of mechanical pre-treatment with screening and de-gritting, primary sedimentation, biological treatment with MBBRs and a chemical coagulation process followed by secondary sedimentation. Presently the plant is running with a load of about 110,000 population equivalents and a flow of around 50,000 m3/day. The MBBR uses a combination of pre-denitrification and post-denitrification with external carbon source addition for nitrogen removal (Rusten & Paulsrud 2009). Wastewater in Norway is cold and very low in soluble chemical oxygen demand (COD), thus all full-scale WWTPs that use biofilm processes for nitrogen removal add an external carbon source.

Figure 1 shows a simplified flow sheet of the pilot. It consisted of three reactors (R) in series and a dedicated settling tank for sludge. Table 1 presents the dimensions and details of the pilot-plant. R1 was anoxic for pre-denitrification (DN). The testing of a small-scale pilot during the earlier phase of this study showed that the process could be simplified for reactors operating in anoxic mode (Rathnaweera et al. 2018). Based on the experiences of that, the R1 reactor was designed with one water level for both the normal operation and the forward wash cycle (FWC). Approximately 62% of the wet volume of R1 was filled with biofilm carriers type BWT 15 (biowater technology), which had been successfully tested in a small pilot in the same project (Rathnaweera et al. 2018). A mixer placed between the water inlet and lower level of the carrier bed was used at low rpm to keep the influent water well mixed before it reached the biofilm carriers. This was done to reduce diffusion limitations because there was no aeration in R1 to facilitate good contact between water, substrates and the biofilm (Ryhiner et al. 1994). R2 and R3 were aerobic for removal of organic matter and for nitrification. R2 and R3 were designed following the original CFIC design described in Rusten et al. (2011), with two different water levels during normal mode and forward flow washing mode. Biofilm carriers type BWT-X were used in the two aerobic reactors. Initial filling fractions, 67% and 71% of the CFIC wet volume, were increased in the middle of April 2017 to 86% and 83%, respectively.

Table 1

Reactor dimensions, biofilm carrier types, filling degrees and protected biofilm surface areas

R1 (Anoxic)R2 (Aerobic)R3 (Aerobic)
Total height × length × width, m 3.01 × 1.70 × 1.70 3.01 × 1.67 × 1.67 3.01 × 1.67 × 1.67 
Wet volume FWC, m3 7.40 7.08 6.93 
Wet volume CFIC-mode, m3 7.40 5.89 5.78 
FWC water depth, m 2.56 2.56 2.51 
CFIC water depth, m 2.56 2.13 2.09 
Biofilm carrier type BWT 15a BWT-Xa BWT-Xa 
Specific biofilm surface area, m2/m3 828 650 650 
Biofilm carrier bulk volume, m3 4.57 (3.94) 5.04 (4.08) 4.80 
Total biofilm surface area, m2 3784 (2561) 3276 (2652) 3120 
Filling fraction at FWC water level, % 61.8 (55.6) 71.2 (58.9) 69.3 
Filling fraction at CFIC water level, % 61.8 (66.9) 85.6 (70.6) 83.0 
R1 (Anoxic)R2 (Aerobic)R3 (Aerobic)
Total height × length × width, m 3.01 × 1.70 × 1.70 3.01 × 1.67 × 1.67 3.01 × 1.67 × 1.67 
Wet volume FWC, m3 7.40 7.08 6.93 
Wet volume CFIC-mode, m3 7.40 5.89 5.78 
FWC water depth, m 2.56 2.56 2.51 
CFIC water depth, m 2.56 2.13 2.09 
Biofilm carrier type BWT 15a BWT-Xa BWT-Xa 
Specific biofilm surface area, m2/m3 828 650 650 
Biofilm carrier bulk volume, m3 4.57 (3.94) 5.04 (4.08) 4.80 
Total biofilm surface area, m2 3784 (2561) 3276 (2652) 3120 
Filling fraction at FWC water level, % 61.8 (55.6) 71.2 (58.9) 69.3 
Filling fraction at CFIC water level, % 61.8 (66.9) 85.6 (70.6) 83.0 

Numbers in parentheses are prior to 10 April 2017.

aBiowater technology AS.

Figure 1

CFIC-MEM pilot setup.

Figure 1

CFIC-MEM pilot setup.

Close modal

The pilot-plant was tested with two wastewater types: from December 2016 to April 2017 with SDWW and from May 2017 to December 2017 with PTWW. Treatment with different influent and recirculation flow rates and dissolved oxygen (DO) concentrations in R2 and R3 were evaluated within the CFIC process. The benefit of this pilot-plant was that the same unit could be operated as a conventional MBBR plant when the water level in R2 and R3 was always kept at the maximum (FWC) height. The pilot was run as MBBR with both water types for a short time period, as part of the start-up procedure.

Phosphorus removal ability from the CFIC effluent was studied during November and December 2017. The effluent of the CFIC process was subjected to phosphorus removal with chemical precipitation by polyaluminium chloride (PAX18) followed by a ZeeWeed–10 (ZW10) membrane for particle separation. ZW10 is a test-scale hollow fibre membrane unit with a nominal membrane surface area of 0.93 m2 and 0.04 μm average pore size (ZENON 2001).

Oxygen transfer efficiency can be calculated by measuring the off-gas from the biological reactors. Theory and the procedure of off-gas testing is described by van Loosdrecht et al. (2016). Off-gas testing was performed for both CFIC-mode and MBBR-mode in R2. Both influent and recycled water flows were kept at 4.0 m3/h during the tests. Four DO probes were used. Two probes were situated at the centre of the tank, one was at 0.7 m from the bottom and the other was at 1.8 m from the bottom of the tank. Two more probes were placed at the same levels, but at 10–15 cm from one of the corners of the tank. The top of the reactor was covered with a tarp that was taped to the sides of the reactor to minimize any intrusion of air from the outside. A hole in the centre provided an escape for the off-gas. The percentage of oxygen in the off-gas was measured with a Dräger gas sensor, placed in the head-space between the water surface and the tarp.

Influent and recirculation flow, NO3–N, pH, DO, temperature and total suspended solids (TSS) were measured at several places in the pilot using online sensors (Hach Lange, Dusseldorf, Germany). Automatic samplers were used for flow-proportional samples of the pilot-plant influent and the normal mode effluent of R1, R2 and R3. Continuous flow samplers were used for wash-water sampling during the FWC of R1, R2 and R3. TSS was measured by an external laboratory. Whatman GF/C glass fibre filters were used for filtration of samples and analysis of TSS. TSS was analyzed according to Standard Methods SM 2540 D and E (APHA/AWWA/WEF 2005). All the other parameters were analyzed on-site, using Dr Lange test cuvette kits, a thermostat LT200 and a Spectrophotometer DR 2800 (Hach Lange, Dusseldorf, Germany).

Overall results of CFIC testing with the two influent water types are given in Table 2. DO in R2 and R3 were measured online and the measurements were used for automatic regulation of air blower speed. DO set-points for the CFIC process varied from 5 to 6 mg O2/l in R2 and from 2.5 to 8 mg O2/l in R3. It is important to note that this is removal of the CFIC only, prior to any membrane separation.

Table 2

Summary of results from treating SDWW and PTWW in the CFIC pilot-plant

ParameterSDWW
PTWW
AverageMedianAverageMedian
Influent flow, m3/h 2.9 2.5 4.5 5.0 
Recirculation, m3/h 4.7 3.7 5.1 5.0 
Wastewater temperature, °C 9.9 10.4 13.3 13.0 
Influent concentrations 
 TSS, mg/l 325 291 156 140 
 Total COD, mg/l 536 588 263 233 
 Filtered COD, mg/l 96 81 67 68 
 Total N, mg/l 41 40 34 33 
 Filtered total N, mg/l 28 26 21 22 
 NH4–N, mg/l 23 23 17 18 
R3 effluent concentrations 
 TSS, mg/l 69 70 84 53 
 Total COD, mg/l 109 113 133 103 
 Filtered COD, mg/l 32 32 33 33 
 Total N, mg/l 18 17 21 21 
 Filtered total N, mg/l 13 13 14 14 
 NH4–N, mg/l 5.7 5.3 1.4 0.16 
 NOX–N, mg/l 7.9 8.2 11.5 11.4 
Nitrogen removal 
 Total N, % 56 – 38 – 
 Influent total N and effluent total N of filtered sample, % 67 – 58 – 
ParameterSDWW
PTWW
AverageMedianAverageMedian
Influent flow, m3/h 2.9 2.5 4.5 5.0 
Recirculation, m3/h 4.7 3.7 5.1 5.0 
Wastewater temperature, °C 9.9 10.4 13.3 13.0 
Influent concentrations 
 TSS, mg/l 325 291 156 140 
 Total COD, mg/l 536 588 263 233 
 Filtered COD, mg/l 96 81 67 68 
 Total N, mg/l 41 40 34 33 
 Filtered total N, mg/l 28 26 21 22 
 NH4–N, mg/l 23 23 17 18 
R3 effluent concentrations 
 TSS, mg/l 69 70 84 53 
 Total COD, mg/l 109 113 133 103 
 Filtered COD, mg/l 32 32 33 33 
 Total N, mg/l 18 17 21 21 
 Filtered total N, mg/l 13 13 14 14 
 NH4–N, mg/l 5.7 5.3 1.4 0.16 
 NOX–N, mg/l 7.9 8.2 11.5 11.4 
Nitrogen removal 
 Total N, % 56 – 38 – 
 Influent total N and effluent total N of filtered sample, % 67 – 58 – 

With PTWW the CFIC pilot had an average influent flow of 4.5 m3/h and a maximum flow of 6.0 m3/h. This corresponded to an average total empty bed hydraulic retention time (HRT) in the biological process of 4.8 h at the FWC water level and 4.2 h at the CFIC-mode water level. At the maximum influent flow, the HRTs were 3.6 h at the FWC level and 3.2 h at the CFIC-mode level. The high filling fraction of biofilm carriers is why the CFIC process can be so compact.

Denitrification

The difference between SDWW and PTWW is the level of particle separation during the primary treatment of the WWTP. Since the wastewater has not undergone biological treatment, the majority of influent COD of filtered sample (FCOD) can be considered as biodegradable soluble COD (BSCOD). BSCOD is based on the measured FCOD in the influent or in a given reactor, minus the measured FCOD in the effluent from a fully nitrifying reactor (Rusten et al. 1995a, 1995b). The carbon source is the controlling factor of the structure and function of the denitrifying bacterial community (Wang et al. 2017; Yang et al. 2018; Xu et al. 2018). It is recorded that removing a part of the influent particulate organic material by primary treatment does not have a significant effect on denitrification (DN) rates (Newcombe et al. 2011; Razafimanantsoa et al. 2014). Rusten et al. (2016) studied the DN rates with filtered wastewater as a carbon source and concluded that the removal of organic matter with a filter cloth with openings as small as 33 μm had no negative effect on the DN process.

Figure 2 shows the DN rate against loading. DN rate with SDWW was limited by the low influent NOx–N (NOx–N = NO2–N + NO3–N) loading, which resulted in concentrations as low as 0.6–0.8 mg NOx–N/l in the reactor. The average DN rate achieved was 0.19 g NOx–N/m2 per day with 0.23 g NOx–N/m2-d loading, which equals 82% removal. The average C/N ratio in the SDWW was only 1.6 g BSCOD/g total N or 2.3 g BSCOD/g FTN. With a concentration in the reactor <10 mg BSCOD/l the DN rates were both carbon source limited and NOx–N limited (Rusten et al. 1995a). Although the NOx–N loading of PTWW tests was comparatively higher, DN rates are scattered in the graph with an average of 0.24 g NOx–N/m2-d against 0.40 g NOx–N/m2-d loading, which equals 59% removal. The maximum DN rate achieved was 0.36 g NOx–N/m2-d. PTWW had an average C/N ratio of only 1.2 g BSCOD/g total N or 1.9 g BSCOD/g FTN. The very low C/N ratio is one of the reasons for the lower percentage removal. However, pre-DN may also be inhibited by DO in the influent and recirculated water from the nitrifying reactor. A model by Hagedorn-Olsen et al. (1994) indicates that with only 0.3 mg O2/l in the bulk liquid in a denitrifying biofilm reactor the DN rate will be reduced to 65% of its maximum value. The present results, however, agrees with reported pre-DN rates for MBBRs, between 0.3 and 0.6 g NO3–N/m2-d (Rusten et al. 2000; Rusten & Odegaard 2007).

Figure 2

Denitrification in reactor R1 with two water types.

Figure 2

Denitrification in reactor R1 with two water types.

Close modal

Nitrification

R2 and R3 were designed for removal of organic matter and for NH4–N nitrification. Figure 3 shows the nitrification rates of the two reactors. Organic matter loading, the ammonium concentration and the oxygen concentration can limit the nitrification rate (Odegaard 2006). For combined carbon oxidation and nitrification, the DO concentration is particularly important for maintaining the system. Fast-growing carbon oxidation heterotrophs outcompete autotrophic nitrifiers for oxygen. DO concentrations of minimum 4–6 mg O2/l are recommended to achieve simultaneous carbon oxidation and nitrification in biofilms, which is nearly twice the DO required for carbon oxidation alone (Hem et al. 1994b).

Figure 3

Nitrification in reactors: (a) nitrification in R2 and (b) nitrification in R3.

Figure 3

Nitrification in reactors: (a) nitrification in R2 and (b) nitrification in R3.

Close modal

Average DO concentrations during CFIC with SDWW in R2 and R3 were maintained at 5.1 and 5.4 mg O2/l, respectively. Average influent NH4–N concentrations to R2 and R3 were 9.6 and 5.7 mg NH4–N/l. The average nitrification rate was 0.14 g NH4–N/m2-d in R2. The reasons for this low nitrification rate in R2 were partly the very low water temperature, a high particle load and a DO that was too low for combined carbon oxidation and nitrification. Nitrification rates in biofilms typically increase with 9% for each °C increase in temperature, as long as the DO concentrations and organic loads are unchanged, and the rates are not limited by low NH4–N concentrations (Rusten et al. 1995b). In R3 the average nitrification rate was 0.31 g NH4–N/m2-d. Several data points with NH4–N limitation reduced this average nitrification rate. Maximum nitrification rates were 0.4–0.5 g NH4–N/m2-d, which is very good with SDWW under these conditions, compared to previously reported rates in MBBRs (Rusten et al. 2000).

Average DO concentrations with PTWW in R2 and R3 were maintained at 5.9 and 7.2 mg O2/l. Nitrification rates of PTWW in both reactors were limited due to the influent NH4–N limitation. The average nitrification rate of R2 was 0.40 g NH4–N/m2-d, but maximum rates were around 0.6 g NH4–N/m2-d which again, compared to MBBRs, is very good in the first aerobic reactor with combined carbon oxidation and nitrification (Rusten et al. 1995b). Under summer conditions (15 °C wastewater temperature) the full-scale MBBR plant at NRA WWTP (the CFIC test site) showed an average nitrification rate in the first aerobic reactor (R2) of only 0.25 g NH4–N/m2-d. Low rates in CFIC reactor R3 were due to ammonium limitations, with a median concentration of only 0.16 mg NH4–N/l in the reactor.

Overall removal of nitrogen and organic matter

For CFIC testing with PTWW average influent concentrations (see Table 2) to the pilot were 34 ± 13 mg total N/l and 21 ± 6 mg FTN/l, which means about 37% of the TN was in particulate form. With no final solids separation, average removal in the CFIC reactors for SDWW and PTWW was 56 ± 14% and 38 ± 14% TN, respectively. Based on filtered effluent samples, average total N removal was 67 ± 11% for SDWW and 58 ± 12% for PTWW, and average total COD removal was 94% and 87% for the two water types. Considering the very low average C/N ratio of only 2.4 g FCOD/g total N in the SDWW and 2.0 g FCOD/g total N in the PTWW during CFIC testing, these are very good removal efficiencies for a biofilm process with only pre-DN. In a previous test with MBBRs 59% removal of total N was achieved, based on influent total N and effluent FTN, with a significantly higher influent C/N ratio of 3.7 g FCOD/g total N, a higher water temperature (19 °C) and more than 50% higher HRT (Rusten et al. 2016).

Phosphorus removal

Showing the suitability of membrane separation alone with the CFIC process, phosphorus removal by coagulation and membrane separation was very successful. The average TSS concentration of the feed flow to the membrane was 66 mg/l, with 100 mg/l maximum and 8.3 mg/l minimum. The membrane flux was 23 l/m2-h and the wastewater temperature was 11.8 °C. As expected with a nominal membrane pore size of only 0.04 μm the reported permeate TSS was below 1 mg/l. The average total phosphorus (total P) and PO4–P concentrations of the feed flow were 2.06 and 0.43 mg/l, respectively. Figure 4 shows that the coagulant dose plays the main role in total P removal in this water. An aluminium/total P (Al/total P)-molar ratio of at least 4 was necessary to guarantee a permeate concentration below 0.1 mg total P/l, which was equivalent to an average dose of about 7 mg Al/l and 59 ml PAX-18/m3 wastewater.

Figure 4

Effect of coagulant dose.

Figure 4

Effect of coagulant dose.

Close modal

Filter effect and FWC

Highly packed carrier bed limits the free movements inside the reactor and serves as a ‘filter’, to keep the particles and bio-solids inside the filter bed and reduce solids concentrations in the effluent from the CFIC reactor. The intermittent cleaning process removes the loose and loosely attached sludge outside and inside the carriers (Rathnaweera et al. 2016). R1 was washed for either 1 or 1.5 h every 24 h with the fast rotation of a propeller. According to the pilot set-up, R2 and R3 had to be washed together and were washed for either 1 or 1.5 h every 48 h, or for 1.5 h every 24 h. The water level in R2 and R3 was increased to provide sufficient volume for carriers to move freely and the aeration in the reactors was increased. The turbulence produced by the coarse air bubbles facilitated good mixing and removal of biomass via collision of carriers, and the biomass was washed-out from the reactor with the forward flow wash-water.

The volume of water for one wash varied from 4–6 m3 per reactor, depending on the influent flow rate and the washing time. Figure 5 shows an example of effluent TSS variation from 23–26 October, measured using online TSS probes, averaged over 5 min periods. The dark grey line with circles shows the R1 effluent TSS and the light grey line with circles shows the R3 effluent TSS during both normal CFIC process and FWC. R1 washed for 1.5 h every 24 h and R2 + R3 were washed for 1.5 h every 48 h. FWCs are clearly shown by the peaks of the graph. The FWC in reactor R1 was started at 09:00 and reactor R3 started at 11:00.

Figure 5

Online TSS concentrations before, during and after the FWC of R1 and R3 from 23–26 October 2017, when treating primary effluent municipal wastewater.

Figure 5

Online TSS concentrations before, during and after the FWC of R1 and R3 from 23–26 October 2017, when treating primary effluent municipal wastewater.

Close modal

As shown in Figure 5 the majority of the surplus sludge was removed during the FWC of reactor R1, so that the average CFIC effluent concentration from R1 was as low as 79 mg TSS/l. Effluent TSS of both reactors was lower than 100 mg/l during normal operation and increased to up to a maximum of 2,000 mg/l in R1 wash-water and up to about 1,000 mg/l in R3 wash-water.

Table 3 compares the compositions of normal effluent and wash-water of R1 and R3 for the test period from March to December 2017. Indicating a good filtration effect, the TSS concentration of wash-water was 12–13 times higher than the CFIC effluent. R1 wash-water and R1 normal effluent had similar FCOD, FTN and NH4–N concentrations. Although the FTN concentration of normal effluents and wash-water of R3 was similar, NH4–N concentration in wash-water was significantly lower. The results indicate that both wash-water types can be returned as influent to R2 after settling. After a good solids separation, R3 wash-water can even be released to the final effluent.

Table 3

Comparing normal R1 and R3 effluent with R1 and R3 wash-water composition for the entire CFIC test period

ParameterNormal effluent
Wash-water
R1R3R1R3
TSS, mg/l (average) 59 45 686 607 
FCOD, mg/l (average) 35 29 36 35 
FTN, mg/l (average) 15.3 15.2 17.9 14.6 
NH4–N, mg/l (average) 8.7 2.4 8.5 0.61 
ParameterNormal effluent
Wash-water
R1R3R1R3
TSS, mg/l (average) 59 45 686 607 
FCOD, mg/l (average) 35 29 36 35 
FTN, mg/l (average) 15.3 15.2 17.9 14.6 
NH4–N, mg/l (average) 8.7 2.4 8.5 0.61 

Off-gas test

The air diffusers in the pilot-plant were tubes with 4 mm drilled holes. Due to the biofilm carriers slowing down the upward velocity of the air bubbles, good oxygen transfer is achieved with these coarse bubble aerators. Table 4 shows the results of the off-gas tests conducted in CFIC-mode and MBBR-mode. In the CFIC-mode testing was done with three DO set-points in the reactor (6, 3 and 2 mg O2/l). In the MBBR-mode testing was done with two DO set-points (5 and 3 mg O2/l).

Table 4

Measurements of oxygen transfer efficiencies in CFIC-mode and MBBR-mode

ModeDO set-point mg O2/lNDO probes, mg O2/l
Avg.Avg. temp. °CO2-transfer g O2/Nm3-ma
1234
CFIC 5.6 5.3 7.3 7.3 6.4 ± 0.48 16.4 10.7 
3.0 3.5 4.4 4.8 3.9 ± 0.12 16.4 11.6 
2.0 2.6 3.4 4.0 3 ± 0.11 16.4 11.3 
MBBR 5.4 5.1 5.3 5.2 5.3 ± 0.17 16.4 7.5 
2.9 3.2 3.1 3.2 3.1 ± 0.17 16.4 7.4 
ModeDO set-point mg O2/lNDO probes, mg O2/l
Avg.Avg. temp. °CO2-transfer g O2/Nm3-ma
1234
CFIC 5.6 5.3 7.3 7.3 6.4 ± 0.48 16.4 10.7 
3.0 3.5 4.4 4.8 3.9 ± 0.12 16.4 11.6 
2.0 2.6 3.4 4.0 3 ± 0.11 16.4 11.3 
MBBR 5.4 5.1 5.3 5.2 5.3 ± 0.17 16.4 7.5 
2.9 3.2 3.1 3.2 3.1 ± 0.17 16.4 7.4 

N, number of tests.

DO 1: 0.7 m from bottom in corner. DO 2: 0.7 m from bottom in centre. DO 3: 1.8 m from bottom in corner. DO 4: 1.8 m from bottom in centre.

aWith 0 mg O2/l in reactor, 20 °C, 1 atm and field conditions.

Data in the table clearly show that the DO concentrations of CFIC are lower in the bottom part of the reactor and higher in the top part. This is as expected due to the packed carrier bed of CFIC. The oxygen probe that controls the blower is in position 1 (0.7 m from the bottom of tank and close to the corner); therefore, the DO concentrations higher up in the reactor are typically higher than the DO set-point. Indicating the good mixing in the reactor, DO concentration in all four measuring points were almost the same during MBBR testing. The oxygen transfer efficiency is given as g O2/Nm3 of air and m of diffuser depth. This is a value for field conditions (dirty water) but has been calculated against a DO of 0 mg O2/l in the reactor, a temperature of 20 °C and a pressure of 1 atm to compare results obtained at different water levels, different DO concentrations and different temperatures (US EPA 1989).

Since the DO concentration in a CFIC reactor is not uniform, the average DO concentration from the four sensors was used for calculations. The oxygen transfer values of 10.7 g O2/Nm3-m, 11.6 g O2/Nm3-m and 11.3 g O2/Nm3-m for CFIC-mode are as close as we can expect with the one decimal reading available on the Dräger instrument for the oxygen concentration in the off-gas. For 0 mg O2/l in the reactor, 20 °C and 1 atm, the average value for all the tests in CFIC-mode was a specific field oxygen transfer efficiency of 11.2 g O2/Nm3-m. The oxygen transfer efficiency in the MBBR-mode was quite a bit less than in the CFIC-mode and it was calculated as 7.45 g O2/Nm3-m. However, these specific oxygen transfer efficiencies were very close to the numbers typically used for the design of MBBRs and found in previous tests and measurements under field conditions (Hem et al. 1994a; Rusten et al. 1996).

DN rate with SDWW was limited by the influent NOx–N loading while the DN rate with PTWW was limited by the very low C/N ratio and other factors like DO concentration and the temperature. However, the present results agree with pre-DN rates between 0.3 and 0.6 g NO3–N/m2-d reported for biofilm processes. Nitrification in R2 and R3 were limited by low influent NH4–N concentrations. Based on filtered effluent samples, the average total N removal was 67% for SDWW and 58% for PTWW, and the average total COD removal was 94% and 87% for the two water types.

The only parameter significantly influencing the removal of phosphorus was the coagulant dose. Dosing of PAX-18 equivalent to an Al/total P-molar ratio of at least 4 in the feed to the membrane tank was necessary to guarantee a permeate concentration below 0.1 mg total P/l. For PTWW this was equivalent to a dose of about 7 mg Al/l.

FWC of R1 removed most of the surplus sludge from the pilot, indicating a good filtration effect. TSS concentration of wash-water was 12–13 times higher than the CFIC effluent. The results indicate that after settling both wash-water types (R1 wash, and R2 + R3 wash) can be returned as influent to R2. After a good solid separation, R3 wash-water can be released to the final effluent.

Off-gas testing with a fairly shallow pilot-plant confirmed that operation in the CFIC-mode gives considerably higher specific oxygen transfer efficiencies compared to operation in MBBR-mode. Averages of all tests showed that 1.5 times more O2/Nm3 of air and m of diffuser depth was transferred to the water phase in CFIC-mode compared to MBBR-mode.

This project was partially funded by RFFH grant 238995 from the Regional Research Council of the Capital of Norway, NRA WWTP, and Biowater Technology AS, Norway. The authors acknowledge the valuable help from the management and operators at the Nedre Romerike WWTP, Aquateam COWI AS, and Biowater Technology AS, Norway.

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