Abstract

This paper documents the results of 12 months monitoring of an upgraded hybrid MBBR-CAS WWTP. It also targets the assessment of the increment of the hydraulic load on existing treatment units with a zero construction and land cost. The influent flow to the plant was increased from 21,000 m3 d−1 to 30,000 m3 d−1, 40% of the existing CAS reactor volume was used for the MBBR zone with a carrier fill fraction of 47.62% and with Headworks Bio ActiveCell™ 515 used as media; no modifications were made for the primary and secondary tanks. The hybrid reactor showed high removal efficiencies for BOD5, COD and TSS with average effluent values recording 33.00 ± 8.87 mg L−1, 52.90 ± 9.65 mg L−1 and 29.50 ± 6.64 mg L−1 respectively. Nutrient removals in the hybrid modified biological reactor were moderate compared with carbon removal despite the high C/N ratio of 12.33. Findings in this study favor the application of MBBR in the upgrading of existing CAS plants with the plant BOD5 removal efficiency recording an increase of about 5% compared with the plant before upgrade and effluent values well within the legal requirements.

INTRODUCTION

To meet stringent regulatory treated effluent wastewater disposal standards, secondary wastewater treatment is undertaken depending mainly on biological processes. These biological processes can be categorized as either suspended or attached growth of which the activated sludge (CAS) process has been used for decades in many wastewater treatment plants (WWTPs) worldwide. But with the increasing influent wastewater flow and organic load, existing CAS plants have demonstrated many shortcomings. Thus it becomes necessary to contruct new aeration tanks and secondary clarifiers requiring more land area and construction cost.

In the last three decades the moving bed biofilm reactor (MBBR) process has been gaining ground in application in WWTPs since its original development in Norway in the late 80s and early 90s (Ødegaard et al. 1994; Hamoda & Al-Sharekh 2000; Ødegaard 2006; McQuarrie & Boltz 2011; Biswas et al. 2013; di Biase et al. 2019). The MBBR has numerous advantages that mainly include high organic and nutrient removal rates, capability of receiving high hydraulic and organic shock loads, the omission of return sludge, low hydraulic retention time, reduced land area and being easily retrofitted in existing WWTPs (Rusten et al. 1995; Ahmadi et al. 2011; Leix et al. 2016; Ødegaard 2017; Ashkanani et al. 2019). Over the years of its operation, the MBBR system has shown better operational flexibility and control in either single- or two-stage operation (Ødegaard 2017) and the performance and efficiency were found to be highly dependent on wastewater characteristics, temperature, oxygen transfer and type of suspended biofilm carriers (Abtahi et al. 2018; Bering et al. 2018; Ashkanani et al. 2019; Collivignarelli et al. 2019).

The literature indicates the use of MBBR in combination with other different conventional wastewater treatment technologies to upgrade existing WWTPs to receive additional hydraulic and organic loads without the need for new land area and construction cost, thus increasing the environmental footprint of existing WWTPs (Hamoda & Al-Sharekh 2000; Tawfik et al. 2010; Ahmadi et al. 2011; Di Trapani et al. 2013; Hanafy et al. 2019). The use of MBBR in hybrid with CAS or integrated fixed-film activated sludge (IFAS) has been thoroughly documented in the last ten to 20 years with regards operational conditions, temperature, effluent quality, nutrient removal, oxygen transfer, biofilm carriers, biofouling, effect of carbon-to-nitrogen ratio (Di Trapani et al. 2010, 2011; Yang et al. 2014; Mannina et al. 2017b, 2018a, 2018b; Piechna & Żubrowska-Sudoł 2017; Daigger & Boltz 2018; Rodriguez-Sanchez et al. 2018; Guo et al. 2019; Hanafy et al. 2019). COD removal rates of up to 98% were achieved in pilot MBBR studies with a C/N ratio of 5 and 10 and with respective nitrogen and phosporus removal of 66% and 77% (Mannina et al. 2017a, 2017b).

With regards the hybrid MBBR-CAS system, one or two compartments are used prior to the CAS reactor and can be part of the existing reactor. The media are placed freely moving in the MBBR compartment(s) with a specific filling ratio of 30%–60% of reactor volume and media-specific surface area of 100–1,200 m2m−3 as documented by McQuarrie & Boltz (2011) and Lariyah et al. (2016). The media selection in the MBBR is a key parameter in the system design and must be properly chosen to give effective results (Ødegaard et al. 1999). Filling ratios of below 70% with a corresponding specific surface area of 350 m2m−3 yield high treated-effluent efficiency when used for upgrading CAS (Ødegaard 2006). The carriers are mostly made from virgin plastic materials with density nearly equal to that of water, 0.95–0.98 gm cm−3, and are retained inside the reactor by means of a special sieve arrangement at the outlet zone of the reactor.

Consequently, this study aims at assessing the upgrade of Kima 1 wastewater treatment plant in Aswan city using a hybrid MBBR-CAS reactor. The upgrade undertaken at the plant was two-fold: to increase the influent sewage flow and to enhance treated effluent quality, and was monitored for a 12-month period to cover both summer and winter seasons. The plant was monitored under regular local operational conditions without any adjustments for C/N ratio. The parameters mainly investigated were BOD5, COD, TSS, TN, NH4-N, TP and PO4-P. Other parameters measured were for control and flow-up purposes and they include dissolved oxygen (DO), pH and temperature.

MATERIALS & METHODS

General description

Aswan is a touristic city in southern Egypt with an estimated population of about 350,000 capita. The city is served by three conventional activated sludge WWTPs, all located outdoors in the south-eastern side of the city. The first two plants are located adjacent to each other and near Kima fertilizer factory and are called Kima 1 and Kima 2 WWTPs. They both receive domestic wastewater only with the factory being served by a separate WWTP for industrial flow with a capacity of 450 m3 d−1. The capacity of Kima 1 WWTP was 21,000 m3 d−1 and that of Kima 2 is 35,000 m3 d−1 (currently being ugraded to 45,000 m3 d−1). The third plant is newly constructed with a capacity of 40,000 m3 d−1 situated in El-Alaki valley. Kima 1 WWTP receives wastewater from two main pumping stations in Aswan city and the flow from these two pumping stations has increased in recent years to about 30,000 m3 d−1. Thus the need arose to upgrade Kima 1 WWTP to accommodate this increased flow in order to maintain the required effluent standards.

Kima 1 WWTP before upgrading

The original Kima 1 WWTP was a CAS system with three circular primary sedimentation tanks followed by three rectangular aeration tanks then three circular final sedimentation tanks as summarized in Table 1. The Kima 1 WWTP before upgrading was receiving an average daily sewage flow of 21,000 m3 d−1 with an HRT of 5.40 hours. The recycle ratio was 0.45 and the return activated sludge (RAS) flow rate was constant and independent of the wastewater influent to the aeration tank, and this resulted in a continuously varying mixed liquor suspended solids (MLSS) concentration which recorded an average value of 3,000 mg L−1. The hydraulic loading rate (HLR) was 15.56 m3 m−2 d−1 and the volumetric organic loading rate (VOLR) was 1,390 gmBOD5 m−3 d−1. The plant was operating efficiently and giving accepted treated effluent but after the increase in the incoming wastewater the efficiency of the plant dropped and the treated effluent failed to meet the required standards.

Table 1

Summary of unit processes in Kima 1 WWTP before upgrading

Unit ProcessNumber of UnitsDimensions (m)Depth (m)
Primary Sedimentation Tank Φ 20 3.0 
Aeration Tanks L 30 × W 15 3.5 
Final Sedimentation Tank Φ 25 3.0 
Contact Tanks L 20 × W 7.5 2.0 
Thickeners Φ 10 3.0 
Sludge Drying Beds 48 L 20 × W 10 0.3 
Unit ProcessNumber of UnitsDimensions (m)Depth (m)
Primary Sedimentation Tank Φ 20 3.0 
Aeration Tanks L 30 × W 15 3.5 
Final Sedimentation Tank Φ 25 3.0 
Contact Tanks L 20 × W 7.5 2.0 
Thickeners Φ 10 3.0 
Sludge Drying Beds 48 L 20 × W 10 0.3 
The secondary clarifiers were not upgraded and they were checked to indicate their capacity to receive the incoming new flow with regards HRT, SOR and solids loading rate (SLR) using the following equations: 
formula
 
formula
where:
  • SLR = Solids loading rate (kgTSS m−2·h)

  • X = Mixed liquor suspended solids (mg L−1)

  • SOR = Surface overflow rate (m3 m−2·h)

  • Q = Wastewater flow rate (m3 d−1)

  • QR = Recycled activated sludge (m3 d−1)

  • A = Surface area of secondary clarifiers (m2)

Kima 1 WWTP after upgrading

The plant was modified by introducing an MBBR stage in the existing three aeration tanks, occupying 40% of its volume at the inlet zone (Figure 1). The design parameters for the hybrid MBBR-CAS system are as shown in Table 2. The media used in the MBBR zone are Headworks Bio ActiveCell™ 515 media made from virgin polyethylene with a density of 0.95 gm cm−3. The influent primary treated wastewater had the following average values: TSS = 251.56 mg L−1; BOD5 = 358.13 mg L−1; COD = 454.65 mg L−1; total nitrogen (TN) = 42.17 mg L−1; ammonia nitrogen (NH4-N) = 23.71 mg L−1; total phosphorus (TP) = 14.01 mg L−1; orthophosphate (PO4-P) = 7.62 mg L−1. Mixing and oxygen requirements are provided by a medium bubble diffused air system placed at the bottom of each tank. RAS is recircled to the CAS zone only as shown in Figure 1. Air is provided by six air blowers: four in operation and two in standby each with a capacity of 8,000 m3 hr−1 and total dynamic head of 800 mbar.

Table 2

Summary of the hybrid MBBR-CAS system design parameters

ParameterMBBR ZoneCAS Zone
Flow Rate (Qd) [m3 d−130,000 30,000 
Volume (V) [m31,890 2,835 
Hydraulic Retention Time (HRT) [hrs] 1.5 2.3 
Influent BOD after PST (BOD1) [mg L−1350 125 
Effluent BOD after CAS (BOD2) [mg L−1– 30 
Volumetric Organic Loading Rate (VOLR) [gmBOD5 m−3 d−13,500 1,090 
Hydraulic Loading Rate (HLR) [m3 m−2 d−155.55 37.07 
Surface Loading Rate for Mobile Carriers (SLR) [gm m−3 d−115 – 
Area of Mobile Carriers Required [m2450,000 – 
Specific Surface Area of Mobile Carriers [m2 m−3500 – 
Volume of Mobile Carriers [m3900 – 
Biofilm Carrier Fill Fraction [%] 47.62   
Oxygen Requirements [kgO2 kg−1BOD] 1.2 1.0 
Air Requirements [m3Air hr−125,760 32,000 
Recycled Biomass (QR) [m3 d−1– 13,413 
Recycle Ratio (R) – 0.45 
Mixed Liquor Suspended Solids (MLSS) (X1) [mg L−1– 3,000 
Food-to-Microorganism Ratio (F/M) [d−1– 0.2 
ParameterMBBR ZoneCAS Zone
Flow Rate (Qd) [m3 d−130,000 30,000 
Volume (V) [m31,890 2,835 
Hydraulic Retention Time (HRT) [hrs] 1.5 2.3 
Influent BOD after PST (BOD1) [mg L−1350 125 
Effluent BOD after CAS (BOD2) [mg L−1– 30 
Volumetric Organic Loading Rate (VOLR) [gmBOD5 m−3 d−13,500 1,090 
Hydraulic Loading Rate (HLR) [m3 m−2 d−155.55 37.07 
Surface Loading Rate for Mobile Carriers (SLR) [gm m−3 d−115 – 
Area of Mobile Carriers Required [m2450,000 – 
Specific Surface Area of Mobile Carriers [m2 m−3500 – 
Volume of Mobile Carriers [m3900 – 
Biofilm Carrier Fill Fraction [%] 47.62   
Oxygen Requirements [kgO2 kg−1BOD] 1.2 1.0 
Air Requirements [m3Air hr−125,760 32,000 
Recycled Biomass (QR) [m3 d−1– 13,413 
Recycle Ratio (R) – 0.45 
Mixed Liquor Suspended Solids (MLSS) (X1) [mg L−1– 3,000 
Food-to-Microorganism Ratio (F/M) [d−1– 0.2 
Figure 1

Flow diagram for Kima 1 WWTP after upgrading.

Figure 1

Flow diagram for Kima 1 WWTP after upgrading.

Analytical methods

Although this hybrid system setup favors the removal of BOD/COD as discussed in the literature (Ødegaard 2006; McQuarrie & Boltz 2011), ammonium nitrogen and orthophosphate were also monitored. All samples were analyzed using Standard Methods (APHA 2017) for: chemical oxygen demand (COD), biochemical oxygen demand (BOD), total suspended solids (TSS), total nitrogen (TN), ammonium nitrogen (NH4-N), total phosphorus (TP) and orthophosphate (PO4-P). COD and TSS were measured weekly while the remaining parameters were measured bi-weekly. Dissolved oxygen (DO) concentrations, pH values and temperature were recorded daily using a multi-channel analyzer (Topac Consort C932). All the data were statistically analyzed using one-way analysis of variance (ANOVA) using JMP®, Version 13.2.1 (SAS® Institute Inc., Cary, NC, USA) with application of the Tukey–Kramer test for post hoc comparison if needed.

RESULTS AND DISCUSSION

DO concentration, pH value and temperature

DO concentrations were recorded for both MBBR and CAS reactors separately and the average values recorded for both reactors were 4.81 ± 0.31 mg L−1 and 3.85 ± 0.37 mg L−1. No special control arrangements were made for DO concentrations during the monitoring period and it was according to plant operating conditions, and Table 3 shows the average monthly DO values recorded. The DO concentrations and pH were measured daily and sometimes more than once daily as an indication of the stable operation of the plant, while other monitored parameters were measured weekly and bi-weekly. Liquid temperature was measured to determine its effect on system efficiency as Aswan falls in a hot dry region with rainfall precipitation varying 1 mm between the driest and wetted months of the year and the variation of the annual temperature being about 16 °C on average. The lowest temperature was 12 °C recorded towards the end of December while the highest value was 42 °C recorded in the summer months of June, July and August. The pH values recorded ranged from 6.19 to 7.80. Average monthly recorded data for temperature and pH are as shown in Table 3.

Table 3

Average monthly DO concentrations, pH and temperature values

 DO (mg L−1)
pHTemperature (°C)
MBBRCASAll ReactorAll Reactor
January 5.10 ± 0.45 4.46 ± 0.24 6.91 ± 0.12 19.20 ± 2.16 
February 5.06 ± 0.41 4.42 ± 0.21 6.98 ± 0.12 18.82 ± 3.47 
March 5.11 ± 0.45 4.25 ± 0.45 7.01 ± 0.15 21.14 ± 4.57 
April 4.99 ± 0.40 4.31 ± 0.44 7.11 ± 0.24 27.93 ± 4.96 
May 4.98 ± 0.33 4.12 ± 0.35 7.20 ± 0.22 32.87 ± 4.88 
June 4.78 ± 0.18 3.49 ± 0.21 7.00 ± 0.15 33.91 ± 4.40 
July 4.48 ± 0.20 3.20 ± 0.34 7.05 ± 0.22 34.56 ± 4.38 
August 4.42 ± 0.20 3.13 ± 0.42 7.26 ± 0.25 34.69 ± 4.27 
September 4.51 ± 0.32 3.46 ± 0.31 7.38 ± 0.21 33.37 ± 4.41 
October 4.59 ± 0.24 3.49 ± 0.38 7.24 ± 0.28 28.30 ± 4.48 
November 4.73 ± 0.20 3.85 ± 0.39 6.99 ± 0.15 22.11 ± 4.10 
December 4.99 ± 0.33 4.03 ± 0.65 6.74 ± 0.24 18.92 ± 3.32 
 DO (mg L−1)
pHTemperature (°C)
MBBRCASAll ReactorAll Reactor
January 5.10 ± 0.45 4.46 ± 0.24 6.91 ± 0.12 19.20 ± 2.16 
February 5.06 ± 0.41 4.42 ± 0.21 6.98 ± 0.12 18.82 ± 3.47 
March 5.11 ± 0.45 4.25 ± 0.45 7.01 ± 0.15 21.14 ± 4.57 
April 4.99 ± 0.40 4.31 ± 0.44 7.11 ± 0.24 27.93 ± 4.96 
May 4.98 ± 0.33 4.12 ± 0.35 7.20 ± 0.22 32.87 ± 4.88 
June 4.78 ± 0.18 3.49 ± 0.21 7.00 ± 0.15 33.91 ± 4.40 
July 4.48 ± 0.20 3.20 ± 0.34 7.05 ± 0.22 34.56 ± 4.38 
August 4.42 ± 0.20 3.13 ± 0.42 7.26 ± 0.25 34.69 ± 4.27 
September 4.51 ± 0.32 3.46 ± 0.31 7.38 ± 0.21 33.37 ± 4.41 
October 4.59 ± 0.24 3.49 ± 0.38 7.24 ± 0.28 28.30 ± 4.48 
November 4.73 ± 0.20 3.85 ± 0.39 6.99 ± 0.15 22.11 ± 4.10 
December 4.99 ± 0.33 4.03 ± 0.65 6.74 ± 0.24 18.92 ± 3.32 

BOD5 and COD removal

Increasing the influent wastewater flow from 21,000 m3 d−1 to 30,000 m3 d−1 reduced the overall HRT of the biological reactors to 3.80 hours, divided between both reactors as shown in Table 2. While operating at this low HRT the system achieved acceptable removal efficiencies with regards carbon removal as this high organic load favored biomass growth and sludge detention in the hybrid reactors. This agrees with what was documented by Mannina et al. (2017a), as they reported a considerable drop in biomass levels when treating low-strength domestic sewage with a low C/N ratio due to the shortage in the required carbon needed for biomass development and growth. Biomass loss in the upgraded MBBR-CAS reactor was not affected by the increased flow rate (hydraulic load) as biomass accumulation in the MBBR is settler-independent and can be easily coupled with secondary clarifiers. Part of the biomass composed is attached on the plastic media which are retained against escaping the reactor by special strainers at the outlet, and this balances the biomass washout due to the increased per unit area hydraulic load. With regards the CAS zone, the RAS pumps were adjusted to deliver the required return flow and keep the MLSS at the design value within the reactor.

The biological reactors receive primary treated wastewater with a reduction of about 30% of the influent raw wastewater waste strength, and the average recorded influent BOD5 to the reactors during the monitoring period was 358.13 ± 14.66 mg L−1, while the average recorded COD value was 454.65 ± 17.53 mg L−1. Average effluent BOD5 recorded for the MBBR zone from the three tanks was 136.04 ± 16.28 mg L−1, giving a removal percentage of 61.99% ± 4.45%, while after the CAS zone it was 33.00 ± 8.87 mg L−1, yielding a removal percentage of 75.96% ± 5.03%. Similarly the COD values were calculated to yield 176.48 ± 18.15 mg L−1 and 52.40 ± 9.60 mg L−1 after the MBBR and CAS zones respectively. The corresponding removal efficiencies were calculated as 61.09% ± 4.65% and 70.07% ± 5.91% for both zones respectively. The overall removal efficiency of the biological treatment stage was 90.82% ± 2.32% and 88.47% ± 2.06% for BOD5 and COD respectively. The recorded effluent values were below the required effluent standards for disposal to agricultural drains, as shown in Table 4. Also the removal efficiencies recorded in the summer months were slightly higher than those of the winter months by about 5%–10% only and this is due to moderate temperature drop between both seasons which records about 16 °C. Findings recorded during this monitoring period for both parameters were similar to those documented by (Di Trapani et al. 2010, 2011; Ahmadi et al. 2011; Hanafy et al. 2019). Figures 2 and 3 present the influent and effluent values during the monitoring period together with the removal efficiencies for BOD5 and COD.

Table 4

Comparison between influent and effluent values in Kima 1 WWTP before and after upgrading

WWTPstatus Before upgrading
After upgrading
Law 48 for year 1948 requirements (mg L−1)
ParametersAverage influent (mg L−1)Average effluent (mg L−1)Removal efficiency (%)Average effluent (mg L−1)Removal efficiency (%)
BOD5 358.13 51.50 85.62 33.00 90.87 60.00 
COD 454.65 68.35 84.96 52.40 88.47 80.00 
TSS 251.56 22.75 90.96 29.50 88.31 50.00 
TN 42.17 – – 21.25 49.18 N/A 
NH4-N 23.71 – – 12.46 46.18 N/A 
TP 14.01 – – 7.36 46.89 N/A 
PO4-P 7.62 – – 4.44 41.84 N/A 
WWTPstatus Before upgrading
After upgrading
Law 48 for year 1948 requirements (mg L−1)
ParametersAverage influent (mg L−1)Average effluent (mg L−1)Removal efficiency (%)Average effluent (mg L−1)Removal efficiency (%)
BOD5 358.13 51.50 85.62 33.00 90.87 60.00 
COD 454.65 68.35 84.96 52.40 88.47 80.00 
TSS 251.56 22.75 90.96 29.50 88.31 50.00 
TN 42.17 – – 21.25 49.18 N/A 
NH4-N 23.71 – – 12.46 46.18 N/A 
TP 14.01 – – 7.36 46.89 N/A 
PO4-P 7.62 – – 4.44 41.84 N/A 
Figure 2

BOD5 profiles of the Kima 1 upgraded hybrid reactors: (a) influent and effluent values for both MBBR and CAS zones, (b) removal efficiencies for each zone and overall reactor efficiency.

Figure 2

BOD5 profiles of the Kima 1 upgraded hybrid reactors: (a) influent and effluent values for both MBBR and CAS zones, (b) removal efficiencies for each zone and overall reactor efficiency.

Figure 3

COD profiles of the Kima 1 upgraded hybrid reactors: (a) influent and effluent values for both MBBR and CAS zones, (b) removal efficiencies for each zone and overall reactor efficiency.

Figure 3

COD profiles of the Kima 1 upgraded hybrid reactors: (a) influent and effluent values for both MBBR and CAS zones, (b) removal efficiencies for each zone and overall reactor efficiency.

TSS removal

Figure 4 presents the profile of the TSS removal during the monitoring period of the treatment plant. The influent and effluent samples were collected after the primary and secondary sedimentation tanks and the system registered an average TSS removal efficiency of 88.31% ± 2.37%. This removal efficiency reflects average influent and effluent values to the system of 251.56 ± 15.22 mg L−1 and 29.50 ± 6.64 mg L−1 respectively.

Figure 4

TSS profiles of the Kima 1 upgraded hybrid reactors showing influent and effluent values and overall reactor efficiency.

Figure 4

TSS profiles of the Kima 1 upgraded hybrid reactors showing influent and effluent values and overall reactor efficiency.

These output values are very satisfactory compared with the required effluent standards and similar work as documented by Di Trapani et al. (2010, 2011), Tawfik et al. (2010), Ahmadi et al. (2011) and Hanafy et al. (2019). Although the flow influent to the plant was increased from 21,000 m3 d−1 to 30,000 m3 d−1 without any modifications done to the existing secondary clarifiers, they achieved this high removal efficiency. This is attributed to the retention of the solids (biomass) in the MBBR zone together with maintaining the original recycle ratio of 0.45 in the CAS zone and due to the reduced organic loading rate after adding the MBBR zone prior to the CAS zone in the biological reactor.

A check on the secondary clarifier HRT, solids loading rate (SLR) and surface overflow rate (SOR) was done based on the new influent wastewater flow and existing full floor area of the three clarifiers. Details of these existing secondary sedimentation tanks are as shown in Table 1. The overall surface area is 1,472.62 m2 with a volume of 4,417.86 m3 and this gives an HRT of 3.53 hrs and SOR of 20.37 m3 m−2· d. The SLR was calculated using the equations stated in the previous section, resulting in 3.54 kgTSS m−2·h and 3.69 kgTSS m−2 ·h respectively. Both equations give similar results and all the values calculated indicate that the existing secondary sedimentation tanks can safely receive the incoming additional flow and yield high solids removal rates.

Nitrogen removal

With regards nitrogen removal, the hybrid MBBR-CAS reactor system showed moderate nitrogen removal and the results obtained were rather disappointing. The system showed a nitrification efficiency of 49.18% ± 9.83% and 46.59% ± 10.41% for TN and NH4-N respectively. The average influent TN and NH4-N load recorded was 42.17 ± 6.61 mg L−1 and 23.71 ± 3.18 mg L−1 and the average effluent recorded was 21.25 ± 3.72 mg L−1 and 12.46 ± 1.91 mg L−1 for both parameters respectively. The deviation in the effluent efficiency was very high at about 10% on average and this was contrary to what was obtained for other parameters monitored. The system registered a C/N ratio of 12.33, which is considered as a high ratio that favors both carbon and nitrogen removal. Nitrogen removal efficiency in this hybrid system is lower that those obtained by other similar systems as dicussed in the literature (Di Trapani et al. 2010; Mannina et al. 2018b; Hanafy et al. 2019) and this is postulated to be due to the difference in operational conditions and system configuration.

Figure 5(a) presents the influent and effluent values of TN and NH4-N recorded during the monitoring period of the Kima 1 WWTP, while Figure 5(b) shows the removal efficiencies recorded for both parameters. It can be noticed from Figure 5(b) that the removal efficiencies of both TN and NH4-N are very close to each other, together with the high deviation in the percentage removal efficiencies. No definite values for nutrients in treated domestic wastewater are stated in the Egyptian standards, however, they state that nutrient values in the final treated effluent will be individually and separately decided for each WWTP according to the end disposal point or reuse purpose of the treated effluent wastewater.

Figure 5

TN-NH4-N profiles of the Kima 1 upgraded hybrid reactors: (a) influent and effluent values for both MBBR and CAS zones, (b) removal efficiencies for each zone and overall reactor efficiency.

Figure 5

TN-NH4-N profiles of the Kima 1 upgraded hybrid reactors: (a) influent and effluent values for both MBBR and CAS zones, (b) removal efficiencies for each zone and overall reactor efficiency.

Phosphorus removal

Figure 6(a) presents the profiles of the influent and effluent values for the total phosphorus (TP) and ortho-phosphate (PO4-P). Reporting values indicate average influent and effluent TP values of 14.01 ± 1.81 mg L−1 and 7.36 ± 1.31 mg L−1 respectively. Average PO4-P values recorded at the inlet and outlet of the hybrid reactors were 7.62 ± 1.24 mg L−1 and 4.44 ± 1.08 mg L−1 respectively. Both TP and PO4-P reported an overall very poor removal efficiency, as shown in Figure 6(b). The average removal efficiencies were 46.89 ± 9.53% and 41.84 ± 10.15% respectively. These removal rates are very low compared with the C/N ratio (12.33), as compared with similar pilot studies by Mannina et al. (2017a). At a C/N ratio of 10 and by using a special pilot reactor arrangement they recorded a PO4-P removal efficiency of 87%, which dropped to 67% when the C/N ratio was reduced to 5.

Figure 6

TP-PO4-P profiles of the Kima 1 upgraded hybrid reactors: (a) influent and effluent values for both MBBR and CAS zones, (b) removal efficiencies for each zone and overall reactor efficiency.

Figure 6

TP-PO4-P profiles of the Kima 1 upgraded hybrid reactors: (a) influent and effluent values for both MBBR and CAS zones, (b) removal efficiencies for each zone and overall reactor efficiency.

By further reducing the C/N ratio to 2, they found that the removal efficiency dropped to 26%. Also the deviation in the removal efficiency amounts to about 10% on average and from Figure 6(b) the difference between high and low removal efficiencies reaches 30.50% and 32.92% for both TP and PO4. This high variation in removal efficiency reflects the system instability with regards phosphorus removal.

Comparing WWTP efficiency before/after upgrading

Data with regards Kima 1 WWTP before upgrading was obtained from the plant operators. Before upgrading, the CAS system operated with average HLR of 15.56 m3 m−2d−1 which is low compared with the HLRs after upgrade yielding 55.55 m3 m−2d−1, and 37.07 m3 m−2d−1 for the MBBR and CAS respectively. The CAS system in both cases adopted a constant RAS flow with a recycle ratio of 0.45 giving a return sludge flow of 9,450 m3 d−1 and 13,500 m3 d−1 before and after respectively, and the MLSS value was around 3,000 mg L−1.

Only BOD5, COD and TSS were monitored in the plant before upgrading. The influent values for all the three parameters were similar to those recorded during the monitoring period after upgrading and the difference is in the effluent values. Table 4 shows a comparison between the influent and effluent values before and after upgrade and the allowable limits of the effluent quality requirements of law 48 for the year 1948 for the discharge of treated domestic sewage to open agricultural drains or saline water bodies. The removal efficiency of the plant after upgrading was slightly enhanced by about 5% for BOD5 removal and 3.50% for COD while TSS removal dropped by about 2%. But in both cases the treated effluent meets the local legal requirements as stated in the Egyptian standards.

CONCLUSION

The results of the 12 months monitoring of the Kima 1 WWTP after upgrading confirm the efficient performance of the use of a hybrid MBBR-CAS reactor at low HRT and high C/N ratio. The hydraulic load was increased from 21,000 m3 d−1 to 30,000 m3 d−1, 40% of the existing CAS reactor volume was used for the MBBR zone with a carrier fill fraction of 47.62% with Headworks Bio ActiveCell™ 515 used as media and no modifications were made for the primary and secondary clarifiers. The hybrid reactor showed high removal efficiency for carbon with average effluent values of 33.00 ± 8.87 mg L−1 and 52.90 ± 9.65 mg L−1 for both BOD5 and COD respectively. The well-stabilized biomass was satisfactorily removed in the final sedimentation tanks with an average effluent value of 29.50 ± 6.64 mg L−1. All the carbon and solids values measured in the final treated effluent wastewater were in accordance with the requirements of the local standards.

In contrast, nutrient removal showed moderate removal efficiencies and did not exceed the 50% value. For all the parameters measured, the removal efficiency for TN was 49.18% ± 9.83%, for NH4-N was 46.59% ± 10.41%, for TP was 46.89% ± 9.53% and for PO4-P was 41.84% ± 10.15%. The removal efficiencies were low despite the high C/N ratio which favors nutrient removal and this was attributed to the hybrid reactor configuration. DO and pH were maintained within required limits throughout the monitoring period, while temperature readings all the year round did not affect the biological activity, with a slight increase in the removal efficiencies in the summer compared with the winter due to the moderate temperature drop between both seasons.

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