The document describes a novel water treatment process combining forward osmosis (FO) and direct contact membrane distillation (DCMD) to produce potable water from wastewater. FO uses a semipermeable membrane to extract fresh water from wastewater into a saline draw solution. DCMD then uses a hydrophobic membrane to vaporize the fresh water from the heated draw solution, producing distilled water. Experiments on a bench-scale system showed water fluxes in FO and DCMD behaved similarly under varying temperature and concentration conditions. Specific reverse salt flux, or draw solution concentration, was influenced by temperature. Maintaining a constant heater operation provided stable FO and DCMD water fluxes better than varying temperature controls.

1. 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:
𝐽 𝑤 =
𝑑∀
𝑑𝑡
∙ 𝐴 𝑒𝑓𝑓
−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. 𝐴 𝑒𝑓𝑓 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 ∙ 𝐴 𝐹𝑂
−1
g ∙ m−2 ∙ h−1
where
𝑑𝑐 𝑓
𝑑𝑡
is the rate of change in
concentration with respect to time, ∀ 𝑓 is
the volume of the feed tank 3.6 L and
𝐴 𝑒𝑓𝑓 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 flux
𝐽𝑠
𝐽 𝑤
.
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).