Waste management

Before we begin with the technical parts of waste management, I would like to share an article in which Dan McDougall reports from the chaos and filth of Dharavi in Mumbai, where a recycling industry is helping thousands claw a way out of poverty. Find the article here: Waste not, want not in the £700m slum. We can see how waste can be managed with such ingenuity.

Waste management can be broadly divided into three sections:

1. Liquid waste management
2. Gaseous waste management
3. Solid waste management

Waste streams can be managed by processes like:

• Extraction
• Oxidation
• Hydroprocessing

How is it done?

• Waste is first analyzed for valuable products
• Valuable products are recovered by the processes mentioned above
• When solvent is used which recovery, it should:
  • not be soluble in water
  • safe to handle
  • biodegradable
  • safe for humans
  • low cost

Extraction:

Speaking of extraction, it can be:

• Liquid-liquid extraction (physical or chemical)
• Liquid-solid extraction (aka adsorption)

Coming to adsorption, here are some examples of commercial interest:

• Polymeric adsorbents
  • Example: Phenol Amberlite™ XAD™4 polymeric adsorbent is used in several locations around the world to remove phenol from wastewater. Even high concentrations of phenol (20,000 ppm) in wastewater have been effectively treated. The resin’s capacity for phenol increases with increasing phenol concentration. Regeneration of the resin is accomplished in several ways: 1% caustic or solvents such as acetone, methanol and formaldehyde. Acetone is frequently used since most phenol plants also have acetone production.
• Activated carbon bed
• Zeolites: These are microporous aluminosilicate minerals

Hydroprocessing:

Hydroprocessing can be divided into three categories:

1. Hydrogenation
2. Hydrocracking
3. Hydrotreating

Hydroprocessing of nitro compounds:

Nitro compounds can be converted into amino compounds by catalytic transfer hydrogenation (CTH). Catalyst is by and large Pd/C and ammonium formate acts as a hydrogen donor. This is done at a temperature of 80 deg C.

Ammonia formate decomposes to nascent hydrogen and gives out NH3. If you use sodium formate, you’ll get a precipitate which is not desirable. The amino compounds can be taken for wet oxidation, direct use of nitro compounds can cause explosions.

A bit about CTH:

Reduction of organic functional groups can be categorized into (i) addition of hydrogen to unsaturated groups as, for example, in the reduction of ketones to alcohols and (ii) addition of hydrogen across single bonds leading to cleavage of functional groups (hydrogenolysis). Molecular hydrogen, a gas of low molecular weight and therefore high diffusibility, is easily ignited and presents considerable hazards, particularly on the large scale; the use of hydrogen donors obviates these difficulties in that no gas containment is necessary, no pressure vessels are needed, and simple stirring of solutions is usually all that is required.

Hydrogenation of organic molecules is one of the processes most used in the synthetic organic chemistry industry. The most common methodology for this process is catalytic hydrogenation, either under homogeneous or heterogeneous catalysis, involving molecular hydrogen and a transition metal. However, despite being a reaction of proven efficiency, it presents a large drawback related with the handling of hydrogen gas (flammable and explosive). For this reason, the scientific community has been working hard in catalytic transfer hydrogenation, which avoids the use of molecular hydrogen in favor of hydrogen donors (alcohols, diimides, amines, hydrocarbons or formic acid).

Did I miss anything? Ah, yes. A very primitive method of waste management. It is called Incineration. But there are several problems associated with this method and hence is not the preferred one.

Disadvantages of Incineration:

• It causes evolution of HCl
• In presence of aromatics, dioxins are produced. Dioxins are toxic.

Hydroprocessing of halogenated aromatics:

In case of chlorinated aromatics, HCl evolves. This acid needs to be neutralized and is done with NaOH.

HCl + NaOH –> NaCl +H2O

The catalyst used for this purpose usually is Pd since it is famous for dehalogenation reactions while Ru is known for benzene ring saturation.

Did you know? Ru works wonders when in water than alone. No one knows why.

Oxidation:

Oxidation can be biological (aerobic) or chemical (molecular O2 via OH* radicals). Wet air oxidation is one other way of handling waste.  It is highly capital intensive as compared to incineration.

Did you know? Anaerobic processes create methane. Methane gives energy.

Read more:

Wet air oxidation by Prof. V. V. Mahajani