Application of Riverbank Integrated Super-Magnetic Separation in River Water Environment Management

. Riverbank integrated super-magnetic separation (ISMS) played an important role in river water (RW) environment management when the diminishing of suspension is a major task. This paper provides a comprehensive analysis of its operation performance from the perspective of pollutant controlling and carbon mitigation potential using the method proposed by intergovernmental panel on climate change. The results showed that turbidity of discharge from ISMS is maintained below 3 NTU and total phosphate removal above 85%. From ISMS to sludge dewater and final incineration, the net carbon emission can is negative at -0.07 kgCO 2 /m 3 RW thanks to incineration with combined heat and power generation and cement substitute by slag. The indirect carbon emission associated with the use of PAC and PAM contributed 79% of the total emissions which amounted to 0.087 kgCO 2 /m 3 RW. Future work is suggested on accurate dosing adjustment for cost reduction and carbon mitigation.


Introduction
With the emphasis on environmental protection, there are increasing water environment projects aiming at water quality improvement, sensory enhancement and ecological restoration. As an important onsite infrastructure, water treatment facility is an indispensable part of water environment management. In order for controlling the water turbidity, coagulation-sedimentation, magnetic coagulation and integrated super-magnetic separation technologies are often used. In recent years，the ISMS process was proposed in projects including ecological recharge of the Tangxi River [1] in Chaohu Lake, the landscape recharge of the ancient town of Tongli [2] in Wujiang. The ISMS technology combines flocculation, sedimentation and filtration processes to remove magnetic suspensions through the strong magnetic force of permanent magnets. As the suspended matter in river water is not magnetic, magnetic seeds were added along with coagulants and coagulants aids to form tiny flocs with magnetic properties through an agitated micro-flocculation process. Suspended matters trapped by the flocs were then screened out by a super magnetic separation equipment. During the same process, the reductions of organic matter content and total phosphorous in river water were also reported [2]. However, one of the drawbacks of ISMS is high maintainace cost since large quantity of coagulant and coagulant aids were consumed in daily operation. The reagents related indirect carbon emission has risen concerns in re-ports auditing the carbon emission of wastewater treatment plant. [3] Despite the importance of attaining 3060 carbon neutrality goal, there has been few studies on the carbon emission performance of riverbank ISMS facility. Therefore, this study investigated the pollutants reduction performance and conducted a systematic calculation of the carbon emissions of an ISMS riverbank facility located in Jiaxing, Zhejiang with an aim to provide guidance on greenhouse reduction potentials and reference on future process optimization.

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Material and Methods

Accounting Subject
Nanhu Lake is located in downtown Jiaxing, Zhejiang Province, with a perennial water surface area of 0.52 km 2 . A hub for the storage and discharge of major rivers in Jiaxing, Nanhu Lake is the starting and ending intersection of many rivers such as Haiyan Pond, Pinghu Pond, and Jiashan Pond. The water body of South Lake is characterized by high suspended solids content, low transparency (at about 25 cm), and high total phosphorus content that is much higher than that of the Class III standard (lake standard) of the water environment functional area assessment. [4]Due to low transparency, the bottom of Nanhu Lake is basically absent of submerged plant coverage. A short hydraulic residence time and a high daily water exchange volume make it more difficult to settle the suspended particulate matter. Comparing with other techniques, ISMS is more suitable for application in central urban areas like Nanhu Lake because of the advantages of a small land demand, rapid processing speed, high automation, and simple modular installation [5]. As shown in figure 1，the main parts of the ISMS system in our study is composed of 16 standard containers in which dosing system, coagulation reactor, super magnetic separator, magnetic separation drum and sludge dewatering equipment were sat,. The building construction parts are composed of a water pool, a storage tank, a dosing room, a reagents warehouse, etc. The river water treatment capacity of the ISMS system is 20 × 10 4 t/d.

Accounting Boundary
ISMS water purification technology is a physical chemical wastewater treatment process based on magnetic disk separation, which integrates micro coagulation, disk solid-liquid separation and magnetic seed recycling technology. The detailed process is as follows: (1)with the aid of coagulant PAC and PAM, the pollutants in the water was captured by floc in which magnetic seed rendering it magnetism. (2)in the superconducting magnetic separator, the micro magnetic flocs was separated from the water by magnetic field.(3)micro magnetic flocs passed through a magnetic separation drum to separate the magnetic material from the non-magnetic sludge, where the clean magnetic seeds are recycled (recovery rate >98%) while the remaining sludge was drained to the sludge treatment system. The sludge dewatering treatment is made of stacking screw machines where PAM is also added. Dewatered sludge at 80% were then transported and incinerated at XinJiaAiSi (XJAS) power plant where slag was reused as building materials.
The starting point for calculating carbon emissions in this paper is the river water inflow of the ISMS system to dewatered sludge incineration in XJAS power plant. A detailed carbon emission accounting boundary was draw in Figure 1. One m 3 of treated river water (RW) is taken as the accounting unit, and the calculations are categorized as direct carbon emissions, indirect carbon emissions and carbon offsets. Direct carbon emissions refer to greenhouse gas emissions occurring within the boundary, including combustion from stationary and mobile sources. Greenhouse gas commonly considered in the assessment of carbon emissions are CO 2 ,CH 4 , N 2 O, HFCs, PCFs and SF. However, ISMS is a physical chemical treatment technique with little involvement of biological mechanisms. Hence, it is presumed that direct emission only came from transporting in this study while GHG direct emission associating with aerobic decomposition and microbial endogenous respiration are negligible in the river water collection, treatment and sludge dewatering unit. Indirect carbon emissions are material consumption by activities within the boundary but GHG emissions occur outside the boundary, e.g. electricity used in the process from the national grid, chemicals added for coagulation and sludge dewatering. Carbon offsetting are actions to reduce carbon emission, such as recovering heat generated from sludge incineration for electricity generation, and replacing cement raw materials with residual ash for building materials.

Direct Carbon Emission Accounting
The transfer of dewatered sludge inevitably requires vehicle transport, which generates direct CO 2 emissions from the combustion of fossil fuels. Most of the large load trucks are diesel vehicles whose diesel consumption is estimated in terms of transport distance. In this paper, the transport distance is measured as 15 km from Riverbank ISMS site to XinJiaAiSi (XJAS) power plant, and the fuel consumption of a domestic truck with a capacity of 15 t is deemed as 15 L diesel per 100 km. Hence, the diesel consumption mass RL(kg) was calculated as follows:

RL
In which, m is sludge dry mass, t; w is sludge moisture content,%; M is unit load capacity, t; L is transport distance, km; AVG is fuel consumption per 100km, L/(100km); is diesel density, 0.84 kg/L. Hence, the direct carbon emission is given by: In which, E , is the direct carbon emission of transport (calculated as CO 2 ), kg; RZ is calorific value of diesel, 43.33 GJ/t; C is carbon content per calorific value of diesel(calculated as C),20.2 t/TJ;α is carbon oxidation rates of diesel, 98%; 44/12 is ratio of relative molecular masses of CO 2 versus C.

Indirect Carbon Emission Accounting
The calculation of indirect emissions used the emission factor method in which carbon mission is calculated by multiplying activity data by an emission factor. (3) In which, E is carbon mission of an activity, kgCO 2 ; D is the amount of electricity or reagent consumed. The consumption of heat is not considered in this study. EF is emission factors which are experience derived factors specifying the amount of a CO 2 generated per unit amount of an activity. Detailed emission factors of electricity or reagent in this study is shown in table 1.

Energy recovery from incineration
The incineration facilities in this study are equipped with energy recovery system. Hence, R In which, R is the carbon offsetting from the combined heat and power generation of incineration; CH is the cogeneration efficiency of combined heat and power generation, 2467 kWh/t; is the emission factors of electricity, 0.5839 kg CO 2 /kWh.

Building material substitute by slag
Slag is solid waste from incineration whose main ingredients are SiO 2 , Al 2 O 3 and Fe 2 O 3 which is similar to that of silicate cement. Hence, R In which, R is the carbon offsetting from silicate cement substitute by slag, kg; is the slag production rate, 17.5% [9]; is the emission factors of silicate cement,0.97 CO 2 kg/kg [10].

Pollutants removal efficiency evaluation
The ISMS system at the riverbank of Nanhu Lake was built at early 2022 with a major purpose to reduce river suspensions. Water quality measurement was done from Feb 2022 till April 2023. Turbidity reduction efficiency of ISMS system at a riverbank of Nanhu Lake was shown in figure 2 that outflow turbidity was less than 3 NTU with a removal rate between 91~98%. Overall, the higher inflow turbidity, the higher turbidity removal rate can be expected. Fig. 2. Turbidity reduction efficiency of ISMS system at the riverbank of Nanhu, Jiaxing As shown in figure 3, total phosphorous removal rate was between 88~93% with an average of 91%. The river solid suspensions and phosphorous removal performance in our study is considerable better than those of previous researches [2,11] in which mean values for suspended solid and total phosphorous removal rate are around 80% and 70% respectively.

GHG Emission assessment
A substantial amount of river water is pumped into the ISMS system at 52.3×10 6 m 3 /a while dewatered sludge was transported at 20796 t/a. A sludge water rat at 0.0005 t/m 3 is used when calculate the GHG emission associated with the sludge dewatering, transport and final treatment. Overall, the net GHG emission in our study is negative at -0.07 kgCO 2 /m 3 RW which is mostly attributed to the carbon offsetting by CHP at the XJAS power plant. The direct emissions are negligible while indirect emissions were sum to 0.087 kgCO 2 /m 3 RW. The consumptions of PAC, PAM and electricity account for 53%, 26% and 21% of contrition to indirect GHG emissions. The addition of PAC and PAM is essential in the coagulation and sludge dewatering treatment process. The chemicals dosages were from daily operation logging and showed in table 2 which are compared with figures from other references. There were two process units where PAM was used. The quantity of PAM at the sludge drying process was 0.5% of the dry weight of the sludge which falls into the recommend range of 0.1%~ 0.5% in Guideline on Best Available Technologies of Pollution Prevention and Control for Treatment and Disposal of Sludge from Municipal Wastewater Treatment Plant (on Trial) [13]. As shown in table 2, the dosage in magnetic coagulation is 0.5 g/m 3 RW which is at the lower end range of 0.5~2 g/m 3 RW as specified in T/CECS 636-2019 Technical specification for magnetic medium coagulation and sedimentation wastewater treatment by China Association for Engineering Construction Standardization. In T/CECS 636-2019, the dosage of PAC is not specified and to be determined by experiments.

Conclusions
ISMS is an effective onsite technology in the removal of solid suspension and total phosphorous in river. By investigating operational data of a typical riverbank ISMS facility of Nanhu Lake, it is evident that coagulant and coagulant aids played a major role in pollutant removal. In terms of carbon emission, it is discovered that the indirect carbon emission with coagulant and coagulant aids amount to 0.069 kgCO 2 /m 3 RW which accounting for around 80% of total emission. The overall carbon emission is negative -0.07 kgCO 2 /m 3 RW with the aid of carbon offsetting pathway by sludge inclination and slag reuse.
The possibility of reagents overdosing in the daily operation is not to be overlooked and a new balance point is to be determined between discharge water quality and coagulant dosage. Future studies concentrating on comprehensive water quality data-logging and accurate dosing system is worthy of exploration in terms of operation cost reduction and carbon mitigation.