With water treatment and wastewater treatment we purify (waste)water.
Such purification will never end-up with 100% purified water only.
We will also get ‘byproducts’ containing the removed components.
For simplification we will address 2 main types below, classified on solids:
A sludge
About suspended solids (TSS)
originated from several processes like:
- filtration (backwash or concentrate flows)
- settling tanks (underflow)
- flotation techniques (overflow)
- bioreactors (wasted biomass)
treatment like: separation (water removal), conversion (biogas and nutrients recovery)
A brine
About dissolved solids (TDS)
originated from several processes like:
- membrane filtration (specially RO)
- ion exchange
- electrodeionisation
treatment like: ‘nothing’ (discharge on sewer), concentration, evaporation, zero liquid discharge (ZLD).
Sludge/Biomass treatment
Sludge treatment is focused on reduction of weight and volume basically. On reducing potential health risks as well. More sustainably, it’s driven by possibilities for biogas production and nutrients recovery.
The choice of a sludge treatment method depends on the type and volume of sludge generated. On comparison of treatment costs required for available disposal options as well. Air-drying and composting may be attractive to rural communities. Limited land availability may make digestion and dewatering preferable for cities. Sustainable drivers may encourage possibilities for energy and nutrients recovery.
Energy may be recovered from sludge through biogas production during anaerobic digestion or through incineration of dried sludge.
In our focus on sludge treatment we basically define it as a biodegradable sludge resulted from wastewater treatment plants. A sludge (or biomass in general) that could simply be thickened and dewatered before disposal. More interestingly it has potential for biogas production and nutrients recovery.
In the figure we have displayed a simplified overall process of sludge treatment covering 3 focus aspects:
- Production of biogas
- Recovery of nutrients (and more)
- Disposal. Target: reduction of weight and volume
Production of biogas
Many sludges (including added external biomass sources) contain serious amounts of organic matter. Such organics can be digested in anaerobic biological processes. See technology biological treatment.
For improved efficiency of such processes it’s of interest to reduce the amount of water first. To increase the amount of suspended solids till for example above 5% first. Still containing over 90% of water in most cases!
Produced biogas must to be treated mostly. Specially to remove sulfur, moisture and sometimes even CO2 as well.
Nutrients recovery
After digestion the digestate (sludge) will contain much less carbon (COD/BOD), but still almost all nitrogen and phosphorus. Only a small part of such N and P will be separated in the dewatering, ending in the digestate cake. The centrate (water) on the other hand, basically will be returned to the WWTP. To unload the WWTP and for reasons of recuperation, techniques can be used to remove and recuperate N and P first. To make furtilisers from it for example.
Disposal
In the end, a sludge or digestate flow will remain to be disposed. Destinations could be agriculture, landfill, incineration, composting, methanisation, and many more. In any case volume and weight reduction is of interest to reduce costs and or to make it ‘logistical feasible’.
Brine treatment
“Utility service”
Companies to produce their process water. With relatively small flows of brine from softeners, demi and RO plants.
Basically not of interest to treat brines (locally).
Therefor to dispose on the sewer, to dilute in the wastewater, to be treated in the wastewater treatment plant.
Mostly the final effluent is still in line with salinity discharge limits (if applicable anyhow).
“Core business”
Desalintation plants from sea or brackish water.
Huge amounts of brine (upto 50% of feed water!) will remain.
Often ‘giving back’ to the sea doesn’t give negative impact at all!? While on the bigger picture you could say that the sea water quality will not change.
But is it sustainable?
How about chemicals used to prevent membranes from scaling/fouling/clogging? How about chemicals to clean the membranes again?
In any case we could consider some treatment. In line with sludge treatment, the same kind of questions could be asked:
- Minimise it’s volume?
- Recuperate valuable components from it?
Good housekeeping
- Improved processes for increased recovery = produce less brine?
- Keep with cleaning chemicals polluted batches of water separate?
Concentration
- Membrane technology. Upto 10% TDS brine?
- Evaporation. Upto 25% TDS brine?
- Crystallisation. Upto solids!?
And then? What kind of disposal? For external reuse or recuperation?
Recuperation
- Chemistry. What is brine? Mostly chloride, sodium and magnesium? Which (chemical) products or liquids could we extract from it?
