Desalination Membrane Management

Abstract

With growing market of membrane technologies the disposal of these spent modules going to be serious issue especially for water industry. Review of status of technologies is briefly highlighted. Keeping this in mind, the various schemes/protocols can be planned and accordingly exploratory studies have been initiated using AOP based primary techniques such as hydro thermal processes. This chapter presents both the open literature and experimental studies related to spent desalination membranes.

1. Introduction

To meet the increasing demand for fresh water under growing environmental awareness and constraints, necessity of desalination techniques are being felt strongly. Potable water by desalination can be produced either by thermal process or membrane processes. Membrane application is an emerging area of interest, as membrane processes operate at ambient temperature and offer one step separation for dissolved constituents on molecular level. Reverse Osmosis (RO) is long established as a large scale industrial membrane process. According to International Desalination Association (IDA) report, the cumulative global installed capacity is now 92.5 million m³/day with 19,372 Reverse Osmosis (RO) plants around the world. By 30th June 2018, this number has increased to 20,000. The large desalination market in recent years has resulted in increased waste generation associated with this technology, which has led to the disposal of more than 840,000 End-of-Life (EoL) membranes (>14,000 tonne/year) every year worldwide. Literature shows that these membranes can be repaired in order to reuse them for secondary and tertiary purposes. Figure 1 shows Installed and projected desalination capacity including mostly applied process. Wide variety of polymers such as cellulose acetate; poly acrylonitrile; polyamide and polysulfones are used for RO modules and ultra filtration processes [1].

Figure 1.

Installed and projected desalination capacity including mostly applied process.

In most of the cases membranes are deployed in spiral configuration. Depending on the process and operating conditions, these modules have design life and needs to be replaced after 3 to 5 years. This generates lot of spent RO module as waste. With growing market of membrane technologies the disposal of these spent modules going to be serious issues. Review of status of technologies is briefly highlighted. Keeping this in mind, the various schemes/protocols [2] can be planned and accordingly exploratory studies have been initiated on primary techniques which are based on hydro thermal processes.

Hence eco-friendly disposal of spent membranes is an important issue for desalination industry. In our initial approach, we have carried out lab scale studies to study the various hydrothermal process techniques on mineralisation of polyamide thin film composite membrane and as well as poly sulfone membrane. The low temperature AOP processes requires mild chemical duty conditions and in turn helps in bringing down the capital costs of waste treatment plants.

2. Status of processes technologies for solid forms of wastes

The waste treatment status in India is typically as shown in Figure 2. The spent desalination membranes being in solid form of waste, the present section of the chapter reviews the status of technologies currently deployed.

Figure 2.

Waste treatment status in India.

The salient features of some of the existing industrial practices in nuclear industry, as well as other industries are presented in Table 1 below.

Sl. No.	Location	Technical data	Remarks
A)	Immobalisation in cement
1	Ringhals Nuclear power station Sweden (1975)	Mixing of dewatered resin with cement (14% w/w) in prefabricated mould 2000 moulds generated	Sp. activity of resins; 3.7x10e2 to 3.7x10e4 G.Bq/cu.mt
2	Federal Republic of Germany (1981)	400 l steel drum mixer 2–3 drums/hr	Do
B)	Immobalisation in Bitumen
3	Barseback bituminisation unit, Sweden (1975)	Feed rate of thin film evaporator is 90 kg/hr. for slurry and 33 kg/hr. for bitumen Op. Temperature is 160°C Capacity 2 drums/day
4	Finland (1979)	Drying in conical steel dryer heated by steam and then mixed with bitumen at 50% w/w Op. Temperature 135°C Capacity 100L/hr
5	Switzerland (1978)	Extruder of 120 L/hr
6	Chalk river, Canada (1976)	Thin film evaporator of 100L/hr
C)	Immobalisation in polymers
7	Chooz immobalisation unit, France (1981)	Polyester in 200 L drum mixing 1–2 drums/day Ambient temperature	Sp. activity of resin 1.85x10e4 G.Bq/cu.mt
8	Fama mobile plant, FRG (1976)	Polystyrene in 110 L drum mixing 1–2 cu.mt/day Ambient temperature
9	Mobile plant, USA(1982)	Modified vinyl ester mixing in container of 4.5 cu.mt
10	Narora, India (1996)	Polyester in modified drum mixing after dewatering
D)	Incineration of spent resin
11	Nine mile island, USA	Fluid bed incineration of capacity 12 kg/hr. Off-gas treatment by scrubber, demister, HEPA, iodine absorber
E)	Green chemistry of PET bottles
	IBM Almaden Research Center, San Jose, CA, USA	PET organocatalytic depolymerisation for chemical recycling. Heating at 190°C with catalyst and ethylene glycol Reactants and catalysts can be recycled many times	Research with KACST and Stanford university
F)	Nylon-6 depolymerisation
	Shangai University China	Water medium at temperatures of 553 K to 603 K and pressures of 6.4 to 12.8 MPa Reaction activation energy is 77.38 KJ/mol	Research

Table 1.

The features of the existing industrial practices.

In recent times, several Hydro Thermal Processes (HTPs) have emerged as eco-friendly alternatives to incineration. Some of these processes are Wet Oxidation; Photo Oxidation and Wet Air Oxidation. The HTPs help in complete mineralisation of the organic wastes and forms CO₂ and H₂O. Sources for oxidising the species are air, oxygen, ozone and H₂O₂. The basic chemical reaction in HTPs is

$$\mathrm{CatalystOrganicsubstrate}+\mathrm{Oxidant}========\to {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{Mineralacid}/\mathrm{salt}$$

Wet Oxidation is a mineralisation process employing powerful oxidants such as Hydrogen peroxide and Ozone [3]. The reaction occurs in aqueous medium at a maximum temperature of 100°C under atmospheric pressure. Use of Hydrogen peroxide for concentrated organic waste mineralisation leads to large volumes of secondary aqueous radioactive waste.

The aqueous streams with small concentrations of dissolved organics are being treated using photo oxidations methods, while Wet Air Oxidations is attractive option for concentrated organic wastes [4]. In this backdrop, it is interesting to investigate some of these concepts for mineralisation of desalination membranes further.

3. Desalination membranes and research needs

3.1 Desalination membranes

Desalination membranes are started with the cellulose acetate based materials initially. But due to their low compatibility for certain conditions such as high temperature and pH, better membranes based on aromatic polyamide became popular in later stages. To meet the large flow requirements, Thin Film composites were developed. TFCs are prepared by interfacial polymerisation on surface of porous support and operate in wide range of pH. The typical section view and surface chemistry of TFCs are as shown in Figure 3 below.

Figure 3.

Cross sectional view and surface chemistry of typical desalination membranes.

3.2 Research needs

Various process streams in front-end/back-end of the nuclear fuel cycle are being treated at present by conventional unit operations. With available spectrum of membrane technologies today, it is pertinent to deploy these technologies either as stand-alone or as integrated processes or hybridised with conventional processes for selective separation of active species, particularly in low active process streams/wastes in addition to desalination purpose. Membranes have a definite life and deteriorates thereafter and hence unable to offer sustained quantity and quality output as desired by design. Disposal of the used membranes, hitherto not attended and cared for could prove to be a major bottleneck in the propagation of mass use of membranes as normal incineration like many other organic wastes could prove to be difficult due to complexity of off-gas treatment and generation of large volume of secondary aqueous wastes and their management. Strategies needs to be developed based on Best Available Technologies (BAT). The various polymer recycling techniques are shown in Figure 4 below.

Figure 4.

Polymer recycling techniques.

4. Low temperature AOP process studies

In order to have a better insight into the process of depolymerisation of PA-TFC, the various components of the TFC membranes has been subjected to different hydro thermal process techniques and studies were performed at a constant temperature in the range of 50 to 75°C and under both static and dynamic conditions. In each run, tokens of size 20mmx20 mm with reaction solution were placed into the reactor. The set point was adjusted to a desired value, and the operation was started. At the end of run, the solution left for cooling and tokens are removed and collected for further analysis.

4.1 Effect of stirring on hydrothermal experiments (HTP-2 process technique) on unirradiated PA-TFC for different durations

Hydrothermal reaction by alkaline hydrolysis is heterogeneous two phase reaction. The intensity of turbulence in the liquid phase would decide the extent of resistance offered by the interface between the phases. Hydrothermal reaction was carried out using 50% caustic solution using polyamide TFC tokens. The reaction was carried out both under static and dynamic conditions and observed results are shown in Figure 5. There is an improvement in degree of depolymerisation of more than 50% under stirring speed of 330 RPM at temperature of 50°C.

Figure 5.

Effect of stirring on HTP-2 process technique.

4.2 Effect of concentration of reaction media at 50°C

Hydrothermal reaction was carried out using 4–50% caustic solution for polyester tokens. The reaction was carried out at temperature of 50°C, under dynamic conditions, using un-irradiated polyester tokens. The observations at 50°C temperature are shown in Figure 6. There is an improvement in degree of depolyemrisation of more than 100% with 50% and 10% alkali concentrations. The DODP of up to 100% could be observed with 10% and 50% alkali concentrations. It is planned to carry out further studies to explore the possibility of recycling of spent polymers through selective chemical treatments.

Figure 6.

Effect of concentration of reaction media on HTP-2 process technique at 50 deg. C.

4.3 Effect of concentration of reaction media at 75°C

Hydrothermal oxidation by alkaline hydrolysis is heterogeneous two phase reaction. Hydrothermal reaction was carried out using 4–50% caustic solution for polyester tokens. The reaction was carried out at temperature of 75°C under dynamic conditions using un irradiated polyester tokens. The results are shown in Figure 7. There is an improvement in degree of depolymerisation of more than 100% with 50% and 10% concentrations. The DODP of up to 100% could be observed with 10% and 50% concentraions in less than 4 hours.

Figure 7.

Effect of concentration of reaction media on HTP-2 process technique at 75 deg. C.

4.4 Effect of irradiation on HTP-2 process technique

The purpose of this investigation is to see effect of irradiation on hydrothermal oxidation by various process techniques. Hydrothermal reaction was carried out using 4% caustic solution for polyester tokens. The reaction was carried out at temperature of 50°C under dynamic conditions, using both un-irradiated and irradiated polyester tokens. The results are shown in Figure 8. There is an improvement in degree of depolymerisation of more than 100% with irradiation. The DODP of up to 80% could be observed with 4% concentrations. It is planned to carry out further studies to explore the possibility of recycling of spent polymers through selective chemical and non chemical treatments.

Figure 8.

Effect of radiation on HTP-2 process technique.

4.5 Variation of COD values for htp-2 process techniques for desalination membrane fabric

Due to depolymerisation the total organic load will increase in the reaction media depending on the efficiency of process technique adopted. The COD/TOC gives better estimate of the same. The Table 2 shows the variation of observed COD values in ‘ppm’ at an accuracy of ± 5% for hydrothermal process based on alkaline hydrolysis techniques tried on desalination membrane fabric made of polyester. The COD values of up to 120 ppm was observed and shown expected trends.

	Material		Poly ester
	Size mm		20x20
	Media		4% NaOH	No condensor and no refluxing
	pH		8.90
	Volume		50 ml
	Temperature (°C)		50
	Stirring rate RPM		330
Sl. No	Token no	Duration (minutes)	Weight loss (%)	COD (mg/L)	AchD code	Remarks
1		0	0.00	0.00
2	TK1	30	1.50	66.00	Q88
3	TK2	60	9.94	43.00	Q89
4	TK3	90	12.60	61.00	Q90
5	TK4	120	16.34	51.00	Q91
6	TK5	150	16.87	122.00	Q92
7	TK6	180	28.55	124.00	Q93	Colour change and brittleness
8	TK7	240	36.57	113	Q94
9	TK8	300	45.96	88	Q95
10	TK9	360	48.8	107	Q96

Table 2.

Variation of COD values during hydrothermal process technique w.r.t polyester.

4.6 Effect of various other catalysts being studied

The non catalytic wet oxidation experiments were carried out on pure polyamide beads at temperature of 100 °C using potassium permanganate oxidation reagent for various durations. The observations at 50, 75 and 100°C are as shown below in Figure 9.

Figure 9.

Non catalytic wet oxidation observations.

Further catalytic wet oxidation experiments were carried out on pure polyamide beads using 30% Hydrogen peroxide with ferrous sulphate catalyst for various durations. The observations at 50, 75 and 100°C are as shown below Figure 10. Further membrane performance studies are being planned to evaluate flux and salt rejection.

Figure 10.

Catalytic wet oxidation observations.

5. High temperature plasma studies for quaternary recycling

The recovery of plastic’s energy content can be achieved through Quaternary recycling methods. Incineration aiming at the recovery of energy is currently the most effective way to reduce the volume of organic materials. Although polymers are actually high-yielding energy sources, this method has been widely accused as ecologically unacceptable owing to the health risk from air born toxic substances e.g. dioxins (in the case of chlorine containing polymers). To achieve improved and efficient combustion, exploratory studies were carried out using air plasma. The mass reduction factors observed for various components of desalination membranes are as shown in Table 3 below.

Operating conditions
Voltange	160		159	volts
Current	199		200	A
Power	31.84		31.8	kW
Plasma gas flow	30		30	lpm
Additional air flow	50		200	lpm
Duration	15		20	min
Avg temperature	3000	K
For RO-TFC membranes			For BARC water filter
Initial mass		97		800.481 gms
Final residue mass		10.31		6.411 gms
Mass reduction factor		89.37113		99.19911

Table 3.

Air plasma studies for desalination membrane management.

The elemental composition of the residue for trail 1 with RO-TFC membranes are as shown Table 4 below.

Sl. No	Element	Concentration	Remarks
1	Al (%)	0.64 ± 0.05	EDXRF, ICPOES, C/S analyser, ISE, N/O analyser, H determinator
2	Cu (%)	0.61 ± 0.46	Inhomogeneous w.r.t Cu
3	Fe (%)	1.2 ± 0.2	Do
4	Si (%)	1.8 ± 0.1	Do
5	Cl^−	1.45	RSD: 5% for Cl
6	C (%)	7.0 ± 0.1
7	S (%)	3.0 ± 0.2
8	H (%)	0.8	RSD: 7% for H
9	N (ppm)	820	RSD: 5% for O and N
10	O (%)	44	RSD: 5% for O and N; Beyond the linear dynamic range of the technique

Table 4.

Composition of residue sample of air plasma studies.

6. Discussion

Based on Environmental regulations that have to be implemented and keeping in mind the release criteria and development of innovative futuristic radiation processing aspects, BAT (Best Available Technologies) can be foreseen. The tentative proposed logic diagram for managing solid form of wastes such as desalination membranes can be represented as shown in Figure 11 below.

Figure 11.

Logic diagram for spent desalination membrane management.

7. Conclusions

Water industry needs to address the spent membrane management, keeping in mind to provide integrated solution. The AOPs are effective techniques to treat high degree industrial wastes. The aspects of depolymerisation and mineralisation were investigated. The variables studied have improved the degree of depolymerisation. Hybrid systems based on different techniques needs to be developed. These techniques have bright prospect in nearby future due to ongoing research initiatives. Best Available Technologies needs to be explored to supplement the conventional biological and chemical methods.

Acknowledgments

The authors wish to thank Director Chemical Engg Group for giving encouragement to the programme. Thanks are also due to various divisions of BARC for their technical suggestions and discussions. Authors wish to thank HBNI for supporting academic aspects of this programme with DGFS-PhD 2018 scholar M srija.

Conflict of interest

There is no ‘conflict of interest’ issues for this book chapter.