Sludge Reduction: Technologies Integrated in the Wastewater Handling Units

Content Table

Sludge Reduction: Technologies Integrated in the Wastewater Handling Units

Addition of chemical metabolic uncouplers

The process of uncoupled metabolism has been studied since the ’90s for the reduction of sludge production. It can be obtained by using chemical metabolic uncouplers such as chlorinated and nitrated phenols, or 3,3’,4’,5-tetrachlorosalicylanilide (TCS). These molecules diffuse relatively freely through the phospholipid bilayer with a transport rate proportional to the concentration gradient across the cell membrane. Once inside the membrane, the phenolic hydroxyl dissipate the proton gradient which is a driving force for ATP production, resulting in the dissociation between anabolism and catabolism. High concentrations of these metabolic uncouplers are needed to favour a higher energy dissociation and the consequent cell growth reduction. The addition of metabolic uncouplers does not block electron transport along the respiratory chain to oxygen, and therefore the efficiency of substrate removal may remain good in most cases; however some compounds cause a reduction of substrate removal up to 26%.

The process is based on the simple addition of the chemical metabolic uncouplers to the wastewater handling units; the effect of the compounds added occurs directly during contact with activated sludge. Efficiency in the reduction of sludge production depends on the type of compound added and on the dosage.

However, little is known about the effect over long periods in which acclimatisation could play a role, or about the optimal conditions for the process or on the potential negative side-effects caused by these compounds.

Chemical and thermo-chemical hydrolysis

Chemical or thermo-chemical treatments are based on alkaline or acid reagents. Coupling an increase in temperature with a strong change in pH, cell breakage occurs promoting the process of cell lysis-cryptic growth. As compared with the simple thermal treatment, the thermo-chemical treatment has an higher efficiency in sludge solubilisation when applied at the same temperature, however giving additional costs for reagents. Alkaline reagents, such as NaOH, are considered to be more efficient than the acids (HCl or H₂SO₄) and NaOH is effectively the most use reagent. Optimal conditions to induce sludge solubilisation and reduce costs are pH>10, temperature>50-60°C, contact time less than 1 h, since longer time do not improve solubilisation effectively. After the hydrolysis in a contact reactor, the lysate is recirculated in the activated sludge stages for further biodegradation.

The integration of a thermo-chemical treatment in the wastewater handling units at full-scale is rare and as far as we are aware, it is difficult to find successful results in the literature. The reason is the unfavourable economic balance of this application due to the high energy requirement for heating a low-concentrated sludge and the increased reagent dosage.

Enzymatic hydrolysis with added enzymes

Hydrolytic enzymes adsorb to the sludge-substrate and attack the polymeric substances leading to solid solubilisation and biodegradation enhancement. Enzymatic treatment of sludge is based on the mechanisms of solubilisation, cell lysis and cryptic growth. Considering the high presence of proteins, carbohydrates and lipids in the composition of excess sludge, the addition of enzymes such as protease, lipase, cellulase, emicellulase and amylase could be advisable. Commercial enzymes for the hydrolysis of organic matter has been already used for the improvement of sludge biodegradation and reduction or to enhance organic waste degradation, but not always the expected performances are predictable. Many of these enzymatic products are patented and their exact composition is generally confidential; in some cases these mixtures contain other stimulatory nutrients as well as enzymes.

Cations such as Ca²⁺, Mg²⁺ or Fe³⁺have a role in the enzymatic treatment of sludge. The removal of these cations by means of cation-binding agents leads to the disruption of flocs and solid solubilisation, resulting in an increase of the specific surface area available for enzymatic hydrolysis. However, at the moment the dosages indicated for these cation-binding agents are very high and not yet recommended for practical applications at large/industrial scale.

The installation of an enzyme dosage system represents the only investment cost. To date, promising results in sludge reduction are referred often to high enzyme dosages and the viability at full-scale has not been yet completely investigated.

Mechanical disintegration

Several systems have been proposed for the mechanical disintegration of sludge, in which energy is supplied as pressure or rotational/translation movement. The aim is to enhance sludge solubilisation as a consequence of the bacteria cell disintegration and the disaggregation of biological flocs. In general, at low applied energy only floc disintegration is observed, while high energy is required to damage microbial cells.

The mechanical disintegration treatment is integrated in the activated sludge process through the addition of an appropriate unit treating part of the return sludge. Sludge is disintegrated and the lysate obtained is recirculated into the activated sludge reactors. Thus the process is based on the cell lysis-cryptic growth mechanism. The systems proposed for mechanical disintegration are:

· lysis-thickening centrifuge;

· stirred ball mills;

· high pressure homogenisers;

· high pressure jet and collision

· rotor-stator disintegration systems.

These systems differ widely with regards to configuration, operational conditions, level of sludge solubilisation and energy consumption. In the case of low energy applications the floc disaggregation is moderate and this may cause a worsening of sludge settleability, but not in all the cited techniques. In the case of sludge with a high presence of filamentous bacteria (bulking phenomena and foaming) the settleability may be improved due to the separation of structures and bridges among filaments.

Microbial predation

Protozoa and metazoa play an important role in activated sludge processes, thanks to their grazing effect resulting in a clearer effluent. Sludge reduction by predation is based on the loss of energy in the food chain and by the change of part of the sludge from a solid to a liquid or gas. The performance of metazoa in sludge reduction has attracted more attention compared to protozoa.

Despite efforts to control the growth and reproduction of predators in the biological systems, the conclusion is that it is very difficult to manage predators directly within activated sludge. Thus some experiences developed innovative predator-reactors separate from activated sludge stages in order to favour the growth of predators.

Predation can be achieved in a two-stage system: (1) a first aerobic stage (chemostat) with short HRT, which favours fast-growing dispersed bacteria, and (2) a second stage with a longer SRT to favour the growth of predators (activated sludge reactor, biofilm system or MBR). In practice, this two-stage system greatly increases the biological volume and its operational costs, and thus it is generally not feasible to apply for municipal wastewater. Alternatively, an additional specialised predation-reactor integrated in the wastewater handling units can be applied, suitable for the growth of predators. The feasibility at full-scale has not been fully tested in municipal wastewater with nutrient removal.

Oxidation with ozone (ozonation)

The advanced oxidation processes consist of the use of ozone (this section), hydroxide peroxide or chlorine and the combination of various oxidants. These oxidative treatments combined with biological degradation have been demonstrated to be very efficient in sludge reduction, but, generally, the main limitation is their economic feasibility.

The treatment based on ozone (ozonation) for sludge reduction has been proposed since the mid ‘90s, initially integrated in the wastewater handling units. To date, sludge ozonation has been successfully applied at full-scale both in industrial and municipal WWTPs. Sludge ozonation causes floc disintegration, cell lysis, organic matter solubilisation and, to a lesser extent, a partial subsequent oxidation of solubilised organics to carbon dioxide (mineralisation).

Ozonation can be applied to a part of the return sludge or directly to the sludge taken from the activated sludge tanks. The ozonated sludge is then recirculated in the activated sludge stages where cryptic growth occurs at the expense of the released biodegradable organic matter.

The recommended ozone dosage is in the range 0.03-0.05 gO₃/gTSS_produced, which is appropriate to achieve a balance between sludge reduction efficiency and cost. At the moment, sludge ozonation results economically sustainable for WWTPs with large capacity or in the areas where sludge disposal costs are very high, or in the case of operational problems such as sludge foaming and bulking.

Side-stream anaerobic reactor (at ambient temperature)

The integration of an anaerobic reactor (operating at ambient temperature) fed with part of the return sludge in an activated sludge process originates the Oxic-Settling-Anaerobic (OSA) process in which a significant sludge reduction has been demonstrated since the beginning of the ‘90s. The cyclic alternation of aerobic/anaerobic conditions uncouples catabolism and anabolism, which causes a decrease in growth yield favouring sludge reduction. However, the mechanisms causing sludge reduction with the OSA system are not yet fully understood and also other explanations based on cell lysis-cryptic growth have been proposed.

The process known as Cannibal is based on physical treatments (screening, hydrocyclones to separate inert solids) and a biological anaerobic reactor (interchange reactor). The introduction of this process in an existing WWTP requires the addition of a mixed tank, which has to operate without oxygen (only short periodic aeration), at a high biomass concentration, a sufficiently long retention time (SRT of 8-15 d), an interchange rate between 4% and 7%, without any wastewater feeding and maintaining a low redox potential (ORP), set approximately at -250 mV. Field operations indicate that the Cannibal process allows a significant reduction of sludge, but only a few studies have been conducted to fully understand and explain the basic mechanisms causing sludge reduction. Although the OSA system led to an increase of P in the effluent, the Cannibal system seems to contribute to P removal, but up to now the fate of P in the system remains enigmatic.

Thermal treatment

The application of a thermal treatment of sludge (by heating sludge) produces disaggregation of sludge flocs, high level of solubilisation, cell lysis and release of intracellular bound water. The main parameter for thermal treatment is temperature, whilst the duration of treatment has generally less influence, expecially when high temperatures (>100°C) are applied. Several investigations confirmed that the highest sludge solubilisation is obtained around 180°C and higher temperatures do not causes appreciable increase of sludge biodegradability which even may decrease, due to the formation of refractory compounds linked to Maillard reactions. However, also the thermal treatment at T<100°C, integrated in the activated sludge stages, causes a significant reduction of excess sludge production, directly linked to an immediate decrease of biological activity and an increase of maintenance requirement. For sludge reduction by thermal treatment, the sludge is heated by steam and/or by heat exchangers prior to enter a contact reactor; then the lysated sludge is recirculated in the activated sludge system. Thickening before thermal treatment is advisable because higher solid concentration in thickened sludge means a save in energy and contact reactor volumes. SVI decreases with the rise in temperature, since the solubilisation of EPS (hydrated compounds able to absorb huge quantity of water) causes the release of a part of linked water.

Ultrasonic disintegration

The ultrasonic disintegration treatment consists of an ultrasound generator operating at frequencies of 20-40 kHz and in a device, which usually is a sonotrode, to transmit mechanical impulses to the bulk liquid. In the application of ultrasounds, pressure waves lead to cavitation bubbles forming in the liquid phase, which grow and then implode releasing localised high energy (local heating and high pressure), which cause sludge disintegration and, at high energy, the rupture of microbial cells.

The basic mechanism of ultrasonic disintegration is cell lysis-cryptic growth. Since the most important mechanism of ultrasonic disintegration is ultrasonic cavitation, it is advantageous to apply ultrasounds at low frequencies and at high energy levels.

In the scheme of ultrasonic disintegration integrated in the wastewater handling units, a part of the return sludge is treated continuously or in batch mode in a contact reactor equipped with sonotrodes. The subsequent biodegradation of lysate is completed in the activated sludge stage. Among the mechanical disintegration systems, sonication is the most energy hungry. A pre-thickening unit before the ultrasonic disintegration is advisable, to operate at higher solid concentration, which allows energy consumption to be reduced. Although several full-scale applications already exist of ultrasonic disintegration integrated in the sludge handling units, the application in activated sludge systems is rarer, due to economic reasons.