Minimal Liquid Discharge - MLD

For a long time ZLD has been suggested as an environmentally friendly way to help the industry meet with increasingly strict discharge requirements and for recycling their wastewater streams. However ZLD processes are

• technically complex
• very expensive
• not necessarily environmentally friendly due to the additional material and energy they require

So more and more end users, in order to improve their water footprint, are adopting a minimal liquid discharge (MLD) approach to the wastewater treatment problem by using dependable filtration-based technologies that can achieve high water recovery at a fraction of ZLD’s costs.

2. Why MLD?

Taking an MLD approach to your process can help users to significantly minimize their CAPEX and OPEX, since removing the final 5 to 10% of liquid in order to achieve ZLD can prove horrendously costly. In order to understand this better, we’ll take the MD-MVC- Crystallizer from Lenntech’s ZLD page  (Link to ZLD (MD-MVC-Crystallizer)) but this time we’ll start counting the total water recovery from a single pass SWRO (with 45% recovery).

100 m³ feed water →Pretreatment → Single Pass RO (45% recovery) → 100 (1-0.45) = 55 m³ Brine → MD (75% recovery) → 55 x (1-0.75) = 13.75 m³ Brine → MVC (90% recovery) → 13.75 x (1-0.9) = 1,375 m³ Brine → Crystallizer (50% recovery) → 1.375 (1-0.5) = 0.68 m³ Brine → Centrifuge of Belt Press

 The energy required for this scheme is,

100 m³ x 3.5 KWh/m³ (RO) + 55 x 6.57 KWh/m³ (MD) + 13.75 m³ x 14.86 KWh/m³ (MVC) + 1.375 m³ x  50 KWh/m³ (Crystallizer) = 350 KWh (RO) + 361.35 KWh (MD) + 204.33 KWh + 68.75 KWh = 984.43 KWh/ 100m³ feed water

So up to MD the system has recovered [100 m³ x 0.45 = 45 m³ (RO)] + [55 m³ x 0.75 = 41.25 m³ (MD)] = 86.25 m³ permeate at a cost of 350 KWh (RO) + 361.35 KWh (MD) = 711.55 KWh.

From the MVC forward, the system is recovering [(100 – 86.25 =13.75) m³ x 0.9 = 12.375 m³ (MVC)] + [(13.75 - 12.375 = 1.375) m³ x 0.5 = 0.688 m³ (Crystallizer)] = 13.06 m³ permeate at a cost of 201.33 KWh (MVC) + 68.75 KWh (Crystallizer) = 270.08 KWh.

What we end up with is 711.55 KWh for 86.25 m³ permeate (Single RO – MD) and 270.08 KWh for 13.06 m³ permeate (MVC – Crystallizer). That is to say that the (Single RO-MD) is producing permeate at a 711.55 KWh / 86.25 m³ = 8.25 KWh/m³ cost and the (MVC – Crystallizer) is producing permeate at a 270.08 KWh / 13.06 m³ = 20.66 KWh/m³. Let us take a look at this point at Table 1 and its graphical representation Figure 1,

Table 1, Recovery and SEC values of the RO-MD-MVC-Crystallizer water treatment process scheme.

	Recovery (%)	SEC (kWh/m3)
RO	45	3.5
MD	86.25	6.57
MVC	98.625	14.86
Crystallizer	99.313	50

Fig.1, Total water recovery of the RO-MD-MCV-Crystallizer process versus their respective SECs. In the graph are also the values of the energy demand at each stage of the process along with the energy demand/m³ of the MLD  and the ZLD stage. The process reaches a MLD stage right after MD and a ZLD after the Crystallizer.

As we can see, the energy demand for further recovery after MD goes up by a walloping 20.66 KWh/m³ / 8.25 KWh/m³ = 2.50 or 250%! At this point every user must decide if the extra step forward is really worth it. Especially since this part of the ZLD process is going to cost typically 60-70% of the total CAPEX.

ZLD may be useful when tight legislations are present or in water-sensitive regions of the world, when every drop counts but is very economically challenging. The last few steps needed to achieve complete ZLD can nearly double the costs.

An excellent MLD real-life example took place at the General Motors (GM) vehicle assembly plant in San Luis Potosi, Mexico. The plant is located in an arid, remote area with no receiving stream or municipal sewer available to discharge wastewater.  By using a combination of RO technology, a high-rate chemical softening process and other technologies, the plant recovers and re-uses 90% of its tertiary wastewater, with the rest 10% of the liquid waste discharged into adjacent solar ponds for evaporation.

Other technology options such as HPRO, EDR, FO and MD, their combinations and hybrids can also raise high the recovery (70-80%) and they require much less energy than thermal evaporation, reducing the size of the latter and consequently the crystallizer (if ZLD is required)

3. Reduced costs & environmental impact