Trattamento delle acque reflue

Breakthrough Curve in Fixed-Bed Adsorption Column

A breakthrough curve in a fixed-bed adsorption column is a plot of the effluent solute concentration versus time. The column consists of a solid bed where adsorption of pollutants takes place. The concentration of the pollutant changes from maximum to minimum in the adsorption zone.

As water continuously flows through the column, the upper portion of the solid bed becomes saturated with solute, causing the adsorption zone to move down the column like a slowly moving wave. This process is not at a steady state.

When the lower edge of the adsorption zone reaches the bottom of the column, the effluent solute concentration starts to rise rapidly. This point is called the breakthrough. The plot of the effluent solute concentration versus time after the breakthrough is the breakthrough curve.

After the breakthrough, little additional adsorption occurs since the entire bed is approaching equilibrium with the feed. The process ends at the exhaustion point.

In the design of a fixed-bed adsorption column, the breakthrough curve is an important factor to consider.

The length of the adsorption zone (Z_A) represents the minimum bed height needed to produce a low effluent solute concentration. The actual bed height must be greater than this minimum to allow a reasonable operating period among adsorbent regenerations.

As an independent variable, the time is equivalent to the effluent volume.

Fixed-bed Adsorption models:

The simplified model by Michaels is considered. It assumes that an adsorption zone with constant shape and velocity moves through the bed (constant-pattern wave, which is established soon after the initial time of operation), and that the rate of adsorption is controlled by the external resistance.

The time required by the active zone to exit from the column is:

If u is the downward velocity of the adsorption zone:

For any position Z with 0<Z<Z_A, we have:

So that:

The material balance of the pollutant in the liquid phase is expressed by:

where C_B is the bulk concentration and C_E is the equilibrium concentration. If A is the column cross section, the value of

The convective velocity is given by:

By separating the variables of the previous equation:

which can be integrated from 0 to Z to obtain:

A similar integration can be extended from 0 to Z, to get: 157

From the last two equations it is eventually obtained:

This equation represents the generalized profile of the breakthrough curve:

The profile of the breakthrough is independent from mass transfer!

It is important to understand how to calculate the operating line, i.e., the distance between bulk and equilibrium concentration. To do that we visualize the solid to move upwards through the column at a sufficient velocity to maintain a stationary adsorption zone, equal to -u.

Under this assumption of steady-state adsorber with moving solid (not true because liquid, the solid leaving the top will be in equilibrium with the entering liquid, and all the solute will be removed from the effluent liquid at the bottom (see the figure).

158A material balance around the entire fixed-bed reads:

Similarly, a material

balance between the bottom and a position within the adsorption zone

reads:The operating line is represented by the generic material balance:

and can be plotted in a C vs q diagram, together with the equilibrium curve

In summary, the dimensionless breakthrough curve can be calculated by the:

and is independent of the mass transfer coefficient, being only a function of the adsorption

equilibrium isotherm.

On the other hand, the height of the active zone is calculated by the: 159

which depends on both the mass transfer coefficient and the adsorption equilibrium

isotherm. For both calculations, the evaluation of as a function of q is needed. Once

the value of Z has been determined, the height of the fixed bed is designed so as to be at

Aleast 10 times the height of the adsorption zone.

Fixed-bed Adsorption models

1. the Moving Bed adsorption (actual flow of the adsorbent) and Simulated Moving Bed
2. (SMB) adsorptionconnections)

With downflow adsorbers, two or more columns are often used in series to

allow more complete use of adsorptive capacity. When the first column approaches exhaustion, it is removed from service and a regenerated solid bed is added to the sequence.

Downflow beds are sometimes used for both adsorption of organic compounds and for filtration of suspended solids. Dual purpose filter-adsorbers usually have lower capital costs than two separate units. If suspended solids are present in the wastewater, downflow beds should be backwashed periodically.

As a rule of thumb: Q=5-15 m /m h in downflow fixed-bed adsorbers; values of have to be measured in the lab for the specific water.

Main sources of sludges in WWWT (see general flowsheet of a WWWT plant) are:

• settleable solids in raw water and wastewater
• excess biomass from biological processes (purge)
• precipitates from (tertiary) chemical processes

Concentration of these solids are often below 1% by weight, i.e., 1 kg of solid with 99DWkg , so that large volumes of sludge must be handled.

The usual goals of

sludge treatment are:

• to reduce the volume of material for which disposal is required
• to change it to a less offensive (ideally inert) form.

If a sludge is concentrated from 1% (10 g/L) to 3% (30g/L), its volume is reduced by a factor of 3 (from 99 to 33 kg of water). Reducing the water content (dewatering) from 99% to 66% means removing 97 kg of water per kg of solid, with high energy duties.

Reducing the harmfulness requires special treatments (sludge may contain harmful species), to obtain.

As a consequence, sludge handling and disposal typically constitute 25 to 40% of the total operation cost of a WWWT plant.

Some properties of domestic wastewater sludges:

• Typical sludge quantities generated by a domestic wastewater treatment process are 3.0 m3 of sludge (purge) per 1,000 m of water from primary settling, and 19.0 m of activated sludge per 1,000 m of water (secondary settling).
• Sludge viscosities are about 6 cp and 25 cp, respectively.

The chemical composition of sludge is of

interest in evaluating its suitability for by-product use. 161For instance, the fertilizer value of sludge is mainly based on the content of nitrogen, phosphorus and potassium. Nitrogen and phosphorus contents of activated sludges are usually near to 5% (2 to 4% nitrogen, 1 to 3% phosphorus), but potassium level is often below 1%, thus they are low-grade fertilizers. On the other hand, the protein content of activated sludge is about 35%, and could be considered as an animal feed supplement.

The fuel energy value of sludge is remarkable: 4,200 kcal/kg (primary sludge), 3,600 DWkcal/kg (activated sludge), 2,500 kcal/kg (digested sludge). Note that this value refers DW DWonly to the dry weight of solids contained in the sludge: ca 1%. Heating value of diesel fuel is 10,000 kcal/kg.

Sludge Treatment Processes

Selection of treatment processes for sludges depends upon the sludge nature, on environmental factors and ultimate disposal options. Main processes are:

1. Concentration (gravity thickening, flotation,

dynamic cochlear thickener): it increases the concentration of solids by a factor of 4-5 or even more, when using coagulants (polyelectrolytes)2.

Stabilization (composting, aerobic digestion, anaerobic digestion): it converts sludges into a form with higher degradability, lower odor and lower pathogenic organism content. Digestion reduces (not too much) the volume of sludge and produces methane (anaerobic). Aerobic digestion gives fewer operational problems and lower organic concentrations in the supernatant liquid (so-called digestate).

Conditioning: by addition of chemicals (chlorine oxidation, lime Ca(OH) to increase pH above 9.5 over which no existent microorganisms can live) or by heat treatment (pasteurization achieved at 75°C). New: use of Ozone. Now the sludge is inert.

Dewatering (vacuum filtration, centrifugation, filter press, drying beds, lagoons): it reduces the water content to a level (65% to 80%) where the sludges can be handled as damp solids (semi-solids) rather than

as liquids. Vacuum filtration, drying beds, filterpress.

Example of a dynamic cochlear thickener

Example of a filter press for sludge5. (not always convenient)

Drying and oxidation, to reduce water content to a minimum.

It includes:

• Heat drying, to reach a moisture content of 10%. Used to prepare sludge for incineration or for sale as fertilizer. High costs for fuel! High temperatures (up to 650°C). Relatively low nutrient level (limited market as fertilizer!)
• Flashdriers and rotary drum driers (inclined cylinder rotating at low speed)
• Incineration: burning of the organic part, but 20-25% is ashes to be disposed of. The energy released by combustion of the sludge solids may be sufficient to satisfy the energy duties (to heat the wet sludge, evaporate the water and heat the vapour stream up to 1,200°C). Primary and secondary sludge with over 30% solids content have enough fuel value to be self-sustaining. Digested sludges require auxiliary fuel.
• Wet air oxidation: it involves

oxidizing organic sludge in aqueous phase at high temperature and pressure (70 bar at 250°C) with supply of air. With solids concentration larger than 5% the reactor is thermally self-sufficient.

Ultimate disposal of residual sludge (spreading on soil if permitted, landfill, lagoons, ocean no more!). Dried sludge and incineration ashes are often dumped in a sanitary landfill.

Scheme of a sludge bowl centrifuge

Scheme of a sludge incinerator

Rpm: ca 600rpm

In the incinerator, flue gases must be treated in a scrubber with water

Composting (aerobial)

In a composting process organic material undergo aerobic degradation (usually, but sometimes also anaerobic) by microorganisms to produce a sanitary, nuisance-free humus-like material. It consists of five steps:

1. Mixing dewatered sludge with a bulking agent (sawdust, straw, )
2. Aerating the compost pile by air, by mechanical turning, or by both of them to promote the aerobic microorganisms instead of the anaerobic ones.
3. Recovery of the
bulking agent (possibly)
1. Further curing and storage for a suitable holding time to complete the aerobic degradation
2. Final disposal
Aeration is applied:
• to supply oxygen
• to control the composting temperature
• to remove excess moisture
Sludge Aerobic Digestion

As said above, sludge treatment unit has two goals:

1. reducing the flow rate of sludge to dispose
2. stabilizing the sludge (delete its putrescibility) to prevent undesired effects.

Aerobic digestion is treating the secondary (and also primary or mixed) sludge in a way similar to the aerobic wastewater treatment.

The organic part t