Estimation of greenhouse gas (GHG) emissions from natural lagoon wastewater treatment plant: Case of AinTaoujdate-Morocco

. The process of removing organic components from wastewater as BOD 5 through wastewater treatment plants has been proven to be a significant source of greenhouse gas emissions, mainly methane CH 4 , carbon dioxide CO 2 and nitrous oxide N 2 O. The reduction of these emissions has attracted more interest given their major contribution to global warming. This study was able to identify and estimate the amount of methane and CO 2 emissions on a monthly basis by a simple modeling approach and an empirical method (IPCC) for N 2 O emissions, in the case of Ain-Taoujdate wastewater treatment plant, throughout the years 2013, 2018 and 2019. The results showed that anaerobic ponds were the main source of on-site emissions with 66% of total contribution and 33% for facultative ponds, followed by the energy consumption of the pumping station as off-site GHG emissions.


Introduction
Greenhouse gas emissions from anthropogenic sources such as use and production of fossil fuel, industrial and agricultural activities, [1] have been scientifically proven worldwide to be the main cause of global warming. The accumulation of GHGs in the atmosphere has increased rapidly in recent decades and threatens not only humans but also the entire planet's ecosystem. [2] Wastewater treatment plants (WWTP) have been recognized as a major source of greenhouse gases due to their on-site and off-site production of three main gases: carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), with a global warming potential (GWP) of 1, 28 and 265 respectively (IPCC 2014).

[3] [4]
On-site emissions are source of different wastewater treatment procedures such as biological processes (CO2 emissions from aerobic microbiological respiration, N2O source of nitrification and denitrification, CH4 from anaerobic digestion). Methane from wastewater was found to contribute 5% of the global methane emissions. [1] Off-site GhG emissions are from the energy consumption by different units of the station, in this case the power consumption of the WWTP and the pumping station.
Since the launch of the National Program of Sanitation and Wastewater treatment in 2006, Morocco has seen the creation of 140 wastewater treatment plants, which natural lagoon process is the most widely used at the national level, accounting for more than 82% of the WWTP carried out and 56% of the volume of treated wastewater. Aerated lagoon accounts for 2% of WWTP, activated sludge 12%, bacterial beds 2% and the algal channel about 2% of WWTP.
The city of AinTaoujdate has a natural lagoon-type wastewater treatment plant. consisting of pretreatment facilities (screen, sand trap), four anaerobic basins and two Facultative basins, drying beds and an operating building.
Lagoon treatment process was used in our study to better estimate and quantify the sources of On-site emissions (Methane) based on the total BDO5 removed (∆DBO) and Off-site (CO2) emissions from energy consumption [5] for the different scenarios 2013,2018 and 2019, and given the lack of data on analyses of total nitrogen removal (∆TN),nitrous oxide will be estimated annually by an empirical method based on assumptions provided by the 2019 IPCC guidelines.

Location of the wastewater treatment plant
The study area is located 5 km north of the city of Ain-Taoujdate (Fig1.B),and it's located 20 km from the city of Fès and 40 km from the city of Meknès (Fig 1.2d).
The city of AinTaoujdate is characterized by a warm and temperate climate, the temperature varies between a maximum of 35°C in August and a minimum of 4°C in January. The rainfall regime is characterized by heavy rains in autumn and winter and almost no rain in summer. Indeed, the rainfall year is composed of two seasons: a wet season from October to May with a maximum of 295.56mm in March and another dry season from June to September (Fig. 4).
Temperature and precipitation regime are two factors that govern the purification performance of the anaerobic basins, the reduction of BOD5 by the station is more efficient in a hot climate, since sunlight and adequate temperature play a very important role with a rate of 70% in the photosynthesis process.
On the other hand, under a winter season, the ambient temperature of the water becomes inadequate for the living microorganisms, the photosynthesis cycle breaks down, therefore less oxygen, the reduction becomes less effective with a rate of 40%.
Afterward, temperatures also influence the transport and dispersion of GHG; they affect chemical reactions between the constituents of the atmosphere. Precipitation, on the other hand, is involved in the formation, transport and deposition of aerosols. They also define the decomposition process (methane emission) and affect the growth of vegetation that absorbs some GHG and produces allergens (pollens, spores). [6] The current situation of the WWTP shows that it is located far from the city (5 km) and is surrounded in the center by areas of very dense vegetation and agricultural land that are governed by the changing seasons during the year (Fig. 3).

Methodology and methodology
The methodology of this study is based on the "Green House gas protocol" which was launched by the WBCSD (World Business Council for Sustainable Development) and the WRI (World Resources Institute), and serves as a knowledge and quantification tool for GHG emissions.
The calculation of emissions will be done in three steps: Step 1: Setting organizational boundaries the organizational boundary for this study includes the pumping station, the WWTP and the grid from which the energy is being imported. (Fig.6) Sludge drying beds were not included in the scope due to the lack of reliable data on the quantity of sludge extracted.
Step 2: Setting operational boundaries Scope 1: On-site emissions: Three gases (CO2, CH4, N2O) will be considered and calculated. CO2 emissions from the plant are not considered in the IPCC guidelines because Carbon dioxide, produced during biochemical treatment processes, is considered to be of "biogenic" origin, Therefore, its quantification should not be included in the estimation of national emissions. The IPCC suggests that biogenic CO2 emissions should not be included, but some protocols encourage their quantification and inclusion in calculations [8].
A simplified modeling approach was chosen for estimating the direct emissions of each unit of the wastewater treatment plant for the BOD5 [5] removal process, which includes the four aerobic and two facultative basins. (Fig.6) Nitrous oxide emissions from the natural lagoon process are insignificant compared to methane emissions,[9] and will therefore be calculated using the empirical method proposed by the IPCC guidelines.
Scope 2: Off-site emissions: Indirect GHG emissions from imported electricity consumption. (Fig.6) Step 3: Calculate and quantify monthly GHG emissions of the wastewater treatment plant for the years 2013,2018 and 2019. The year 2013 will be taken as a reference year since the station had a good performance in terms of its purification efficiency.

Fig. 4 Organizational boundary of the WWTP
The organizational boundary shown in Figure 6 includes the direct and indirect emissions of the wastewater treatment plant: Pumping station: 3 pumps including one backup pump with a nominal power of 15 Kw, and a total flow rate of 59l/s; Pre-treatment operations, including a stilling basin, a decanting channel equipped with an inclined screen, an electromagnetic flow meter, and a venturi channel; Two flow distributors (FD1&FD2), circular in shape (Diameter = 4m), one for anaerobic basins and one for facultative basins; (not included in the inventory, works as gravitational flow) Four anaerobic Ponds (AP1, AP2, AP3, AP4) with a useful volume of 2,300 m 3 and a useful height of 3m; Two facultative ponds (FP1&FP2) with a volume of 16,500 m 3 and a useful height of 1.5 m; The station is also equipped with a sludge drying bed of Four units with a total surface area of 960 m2, but has been excluded from the boundary due to insufficient data and will therefore not be included.
The default maximum production capacity (B0) of CH4 for domestic wastewater is 0.6 Kg CH4/Kg BOD.

Nitrous oxide (N2O): estimated from
reactions associated with anoxic or anaerobic biological processes through nitrification and denitrification. [11] Step.1: Calculation of total annual amount of nitrogen in wastewater TNDOM :  The factor for industrial and commercial codischarged protein into the sewer system (FIND-COM) will be 1.25 kg N/kg N (source: IPCC 2019 guidelines).
Step.2: Calculation of the emission factor for nitrous oxide EF The default value that has been taken for the EF is 0.001 kg N2O-N/kg N (according to IPCC 2019 guidelines) Step T=degree of utilization treatment/discharge pathway or system EF=emission factor for treatment/discharge pathway or system j, kg N2O -N/kg N

Scope 2: Off-site GhG emissions from the WWTP:
Indirect GHG emissions have been included as the electrical grid from which energy is being imported to supply the WWTP (Sand trap, Screening bar and the Electromagnetic flow meter), and the pumping station. It should be noted that the total emissions in 2019 are provisional and therefore lower than the previous years since the last months (October, November, December) are not included in the study.

Results and discussions
2018 is considered to be the year with the highest methane emissions with a total of 7920 KgCO2e with an average of 781,74 KgCO2e compared to 851,76 KgCO2e in 2013, and 840 KgCO2e in 2019.Should be noticed that the anaerobic basins were the major source of methane than facultative ones, except January and March with a total of 733,83 and 655,2 KgCO2e higher than anaerobic basins. It can be explained by the purification process, which means that during these two months, the wwtp faced some issues such as high organic load flow or aerobic and anaerobic conditions in both basins were not adequate.
Variations in methane emissions vary from year to year depending on the climate, volume load and water flow to be treated, emissions are high during June, July, August, and low during the rainy season The current year recorded a maximum temperature of 32C, 35 C in June and July (Fig. 4) with a maximum methane emission of 733,82 and 629 KgCO2e, this increase in emissions can be explained by the increase in drinking water consumption, due to the arrival of mass of people during summer holidays, therefore a high production of waste water.
The facultative basins were the minor source of methane emissions with an average of 396,52 KgCO2e, meaning that at the exit of the WWTP the treated water becomes a small source of methane compared to the raw water.
The variation in methane emissions is related to the BOD5 removal process, if we have a good efficiency from the plant, there will be more gas emissions and therefore treated water that meets discharge standards, but if we have a low efficiency from the plant, fewer gas emissions will be emitted and therefore non-compliant treated water.
Nitrous oxide (N2O) emissions, estimates were calculated annually by methods provided by the IPCC during the same periods (Tab.11), as analyses for total nitrogen removed (TN) were not performed, knowing that natural lagoon process is considered a minor source of nitrous oxide.   Table.12. Total Electricity consumption and off-site emissions of the plant and the pump station *Source: [14] Estimation of off-site emissions from the pumping station show that the pumping station is the major source (2865,13 KgCO2e) contributing to 57.7% of total emissions (4958,77 KgCO2e) followed by the wastewater treatment plant with a contribution of 42.2% or 2093.64 KgCO2e.
By comparing the electricity consumption of our case natural lagoon-type station with the other types of stations, we can see that the electricity consumption for the bacterial bed-type process is estimated at 2,300 kWh/year for a capacity of 400 IE(inhabitant equivalent) and at 6,400 kWh/year for a capacity of 1000 IE, followed by aerated lagoon, where aeration is the largest energy consuming Unit with a specific energy consumption of about 1.1 to 2 kW/kg BOD5 eliminated and from 100 to 370 Wh/m 3 . [15] Eventually we find the activated sludge process which has been adopted for steps of more than 100,000 inhabitants and which is the most widely used in Morocco with an estimate of 280 to 780 Wh/m 3 when moving from a high to a low activated sludge load. Therefore, the electricity consumption at the level of natural lagoon process is estimated at 10% of the wwtp consumption per activated sludge. [16]

Conclusion and recommendations
The simple modelling approach for direct and indirect estimation has demonstrated accurate results on each unit of the treatment plant, better than the methods proposed by the IPCC guidelines.
These results showed that anaerobic basins accounted for the majority of on-site emissions followed by facultative basins, and the third source was the pumping station.
According to these estimation, the station is experiencing an exponential evolution of GhG emissions during the 2013-2019 period, and this evolution can be explained for several reasons: the increase of population connected to drinking water over the years, thus an increased production of wastewater, the increase of organic load received by the station, the long sludge cleaning period that takes place every 5-10 years, since the sludge retained at the bottom of the basins contribute as an important source of methane and CO2.
To be able to reduce the GHG emissions emitted by the WWTP, it is possible to opt for an aerated lagoon process using clean energies (e.g. Solar panels) which should increase the purification efficiency of the plant with less sludge production and a short cleaning period.