A paradigm shift that utilizes water reuse strategies is necessary as water resources

become more stringent. The system is a unique combination of proven unit

processes: forward osmosis (FO), direct contact membrane distillation (DCMD),

anaerobic membrane bioreactors (AnMBR), Sharon, Annamox, and struvite

precipitation. During FO, the influent wastewater is concentrated by extracting fresh

water into a high-salinity draw solution through a semipermeable membrane. During

DCMD, the draw solution is heated and the fresh water is vaporized through a

hydrophobic porous membrane, producing a potable-quality distillate. FO is used as a

pretreatment step because it operates with little to no hydraulic pressure, rejects a

wide variety of contaminants, and may be less susceptible to membrane fouling [1].

DCMD is selected to desalinate the FO draw solution due to the low-grade heat

stream requirement that will be available from the biogas generated by the AnMBR

process. When compared to traditional treatment systems, FO-MD may have lower

energy consumption, may improve pollutant removal and may increase the ability to

successfully implement “sewer mining” processes.

Jairo Luque Villanueva, Dustin S. Fredricey, Lianna M. Winkler-Prins, and Andrea Achilli*

Environmental Resources Engineering, Humboldt State University - *Corresponding Author: aa2767@humboldt.edu

Paradigm Shift: Waste to Resource through a Novel Forward Osmosis –

Direct Contact Membrane Distillation Treatment Process

Materials and Methods

Background

The authors would like to thank the WaterReuse research foundation for

funding the project, the Environmental Protection Agency for funding Jairo

Luque Villanueva, and the HSU ERE faculty and staff for their continued

support.

A fully-coupled automated FO-DCMD bench-scale system was designed and

constructed. Experiments were conducted to determine the solute flux and water fluxes

through FO and DCMD. The specific solute flux was determined to approximate the

amount salt that must be replenished in the system. Ultimately, these preliminary results

allow for system performance improvements.

Product

water

Membrane distillation

water production

Energy

Nutrient

recovery

Waste-

water

Anaerobic – Anammox

biological wastewater

treatment

Forward osmosis

dewatering unit

• Dual barrier approach is

applicable for potable reuse

• Low-energy, high-quality

forward osmosis (FO) water

recovery process

• Waste heat and energy from

anaerobic biological process

used for membrane distillation

• Low-energy nutrient (N & P)

recovery for beneficial reuse

[1] Cath, T., Childress, A. and Menachem Elimelech. (2006). “Forward

osmosis: Principles, applications, and recent development.” Journal of

Membrane Science. 281, 70-87.

[2] Achilli, A., Cath, T. and Amy E. Childress. (2010). “Selection of inorganic-

based draw solutions for forward osmosis applications.” Journal of Membrane

Science. 364, 233-241.

Figure 1. Low-energy wastewater and water reuse treatment process.

Conclusions

Acknowledgements

References

The FO-DCMD system consists of four tanks, three pumps,

two membrane modules, a heating element, a heat

exchanger, and a proportional control valve (Figures 2 and

3). Other instruments include resistance temperature

detectors (RTDs), conductivity and pH probes, and

rotameters. Flow temperatures in DCMD are controlled by a

heating element and combined heat exchanger and

proportional control valve mechanism. Proportional Integral

Derivative (PID) controls were used in LabVIEW to raise or

maintain the feed temperature for DCMD and regulate the

distillate temperature. Data were collected each second and

averaged at one-minute intervals. The system was tested

with Porifera® and GE® membranes contained in custom-

made membrane modules of acrylic material. A surrogate

wastewater characterizing typical wastewater influent was

developed. Water flux and reverse salt flux were determined

experimentally over >12 hour tests.

Figure 2. Bench-scale FO-

DCMD system

Objectives

Figure 4. FO and DCMD water fluxes with relatively constant

draw solution concentration of (38 g/L- 40g/L).

Figure 5. Specific reverse solute salt flux dependence on

temperature of the draw solution.

Results In Figures 4 and 5, the MD feed is bounded at 50 °C and 30 °C as

upper and lower bounds for the self-adjusting heater PID. While

operating between the bounds, the heater is capable of maintaining a

constant draw solution concentration. The oscillating temperature in the

MD feed affects FO and DCMD water fluxes similarly (Figure 4). The

DCMD process is influenced primarily by temperature variations of the

feed solution. The change in vapor pressure results from the varying

temperature and affects the water flux through the DCMD membrane.

As the temperature is decreased the water flux through DCMD

decreases and the FO draw solution becomes slightly more

concentrated. However, the increase in draw solution concentration is

not large enough to hinder the osmotic gradient across the FO

membrane.

The specific salt flux gives insight into the replenishment demand of the

draw solution and can therefore be utilized in monetary analysis for

operation [2]. For example, a draw solution temperature of 50 °C will

lead to a loss of 0.5 g of NaCl per 1 liter of solution (Figure 5). The

draw solution will require replenishment through dosing or by allowing

the DCMD process to re-concentrate the draw solution.

Constant heat was used instead of a PID control (Figure 6 and 7). The

constant delivery of heat was measured as a percentage of heater

operation. This approach in heating is more realistic for scale-up and

industrial processes rather than varying temperature as in Figures 4

and 5. The results from this experiment give insight into the range of

DCMD water flux with varying feed side temperatures. The relatively

steady operating temperature lends itself to constant FO and DCMD

water flux (Figure 7). The FO flux does not appear to be influenced by

the small variation in draw solution concentration.

Figure 7. DCMD water flux as a function of the feed side

temperature with a constant distillate temperature of 22C.

Figure 7. FO and DCMD water fluxes with constant heater

operation (50% on).

The water flux for the FO and DCMD

processes were determined as follows:

𝐽𝑤 =𝑑∀

𝑑𝑡∙ 𝐴𝑒𝑓𝑓

−1 L ∙ m−2 ∙ h−1

where 𝑑∀

𝑑𝑡 is the rate of change of the DI or

distillate water tank with respect to time

for the FO or DCMD processes,

respectively. 𝐴𝑒𝑓𝑓 is the effective

membrane area for FO (0.01742 m2) or

DCMD (0.01394 m2). Similarly, the

reverse salt flux was determined using

the following relationship:

𝐽𝑠 =𝑑𝑐𝑓

𝑑𝑡∙ ∀𝑓 ∙ 𝐴𝐹𝑂

−1 g ∙ m−2 ∙ h−1

where 𝑑𝑐𝑓

𝑑𝑡 is the rate of change in

concentration with respect to time, ∀𝑓 is

the volume of the feed tank 3.6 L and

𝐴𝑒𝑓𝑓 is the effective area of the FO

membrane. The specific reverse salt flux

g/L is then determined from the ratio of

the reverse salt flux to the water flux𝐽𝑠

𝐽𝑤.

Water and Reverse Salt Flux

Figure 3. The fluid in the two-stage FO-DCMD process is conveyed with a co-current flow

configuration through the membrane cells. In the first stage, fresh water is extracted from the

wastewater with FO. The fresh water then mixes with the draw solution. In the second stage, the

draw solution is desalinated with DCMD. The draw solution concentration, which is diluted in the

first stage, is re concentrated with DCMD as water vapor only permeates into the distillate tank.

50% On

1. Water fluxes in DCMD and FO exhibit similar behavior under

varying temperature and concentration conditions (Figures 4

and 7).

2. The specific reverse salt flux is influenced by temperature

(Figure 5).

3. The heater is capable of maintaining a constant draw solution

concentration while operating under appropriate temperature

bounds. However, constant heating may provide a more

efficient and realistic approach for up-scaling (Figure 6 and 7).