Resource Recovery from Wastewater

    

    

    Figure 1:  Wastewater treatment plants-from reduction approach to recovery approach.
    
    

Energy Scenario

Current water and wastewater treatment schemes in developed countries such as the United States of America and United Kingdom consume up to 5% of the national electric load [5]. This demand is several fold higher when the energy budget for small communities is evaluated [3]. Anaerobic treatment processes are also well known around the same time as the aerated technologies. Due to slow removal rates, the aerated technologies have become superior. In anaerobic treatment process, organic carbon can be partly converted into methane (biogas) and carbon dioxide. Methane can be used for heating applications within the plant operations to reduce operation energy requirements.

Another anaerobic technology that directly converts organic carbon into electricity is called a microbial fuel cell [6,7]. In this process, exoelectrogenic (electron producing) bacteria fed by organic carbon in the anode chamber convert it into carbon dioxide and release electrons which are harvested through an electric circuit connected to an electron accepting cathode chamber. This process can also be modified to remove or recover other valuable chemicals and metals or even produce biofuels [8-10].

Integrating heterotrophic and photosynthetic processes for conversion of organic compounds into the lipid-rich biomass is another efficient approach for biofuel production. This also helps address the energy consumption and environmental emission issues involved in wastewater treatment [2]. When wastewater treatment and biofuel production are combined in a microalgae system, energy returns are more favorable [11,12]. Energy-efficient anammox process can be considered for sludge reduction and nitrogen removal as reported in recent investigations [13,14]. Wastewater influent temperature gradient may be harvested though heat cycles for beneficial use as well. Finally, a small portion of energy may be recovered if the wastewater treatment plant is located geographically at a position that provides adequate hydraulic head for electricity production [3].

Nutrient Scenario

Nitrogen (N) and phosphorous (P) are considered the major nutrients essential for life. Nutrients are required for agriculture, farming, domestic gardens and aquatic environments. The same nutrients in excess quantities can cause environmental issues especially related to aquatic environments. Nutrients are removed in wastewater treatment plants whereas same nutrients are required in other applications often purchased at high costs. The anthropogenic fixation of N from the atmosphere, nearly exclusively from the carbon-intense Haber-Bosch process, surpassed nature’s fixation of N several years ago. The massive release of bio available N to the environment has been referred to as “global fertilization.” While much P also is released into the environment, the global issue related to P is that the world is running out of high-phosphate deposits that are readily mined to feed the global appetite for P [15]. In view of these concerns, nutrient removal from wastewater is not a choice any more. Moreover, wastewater treatment plants must recover nutrients rather than remove them.

Struvite production is a well-known process for many wastewater treatment operations especially for nutrient rich wastewater streams from agricultural, farming and dairy industries [16,17]. Nitrogen and phosphorus can be recovered together through struvite formation. This can be achieved at multiple locations in a wastewater treatment plant such as the waste activated sludge stream, and anaerobic digester effluents. Sludge may be dewatered and incinerated to recover phosphorous. Further, as discussed earlier, nutrients can be recycled by embedding them into new lipid-rich biomass produced in the integrated processes. Biorefiney concepts can be developed to recovered nutrients by employing a variety of microbial and bioelectrochemical systems.

Water Scenario

Water is the most essential commodity for sustainable development. Due to population growth and rapid industrialization, many regions of the world are seeking the option of reusing wastewater effluents for non-potable applications [18,19]. The end use application determines the level of treatment required and the desired water quality. There are several technologies suitable for reclaiming treated wastewater for beneficial uses. These include membrane (microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis) technologies, adsorption (ion exchange, Granular Activated Carbon-GAC) technologies and other physicochemical processes such as advanced oxidation processes including ultraviolet disinfection, ozonation, Fenton reaction and photocatalytic reactions [20]. The level of treatment in terms of pollutant removal capabilities are shown in Table 1. Advanced oxidations processes are implemented to remove the dissolved organic carbon and other emerging contaminants of concern [20,21]. These technologies are feasible in many applications due to high costs.

  

    
Process 
    
Dissolved organic    matter 
    
Trace organics 
    
TDS 
    
Bacteria &    Protozoa 
    
Viruses 
  
  

    
Microfiltration
    
 
    
 
    
 
    
X
    
 
  
  

    
Ultrafiltration
    
 
    
 
    
 
    
X
    
X
  
  

    
Nanofiltration
    
X
    
X
    
X
    
X
    
X
  
  

    
Reverse osmosis
    
X
    
X
    
X
    
X
    
X
  
  

    
Electrodialysis
    
 
    
 
    
X
    
 
    
 
  
  

    
Granular Activated    Carbon (GAC)
    
X
    
X
    
 
    
 
    
 
  
  

    
Ion exchange
    
 
    
X
    
X
    
 
    
 
  
  

    
Advanced oxidation    processes
    
 
    
X
    
 
    
X
    
X
  
  

    
Ultraviolet    disinfection
    
 
    
 
    
 
    
X
    
X
  

Table 1: Advanced water treatment technologies for water reuse applications.