Industrial waste water treatment: the anaerobic alternative

TIBTECH - DECEMBER

1986

standard conditions. Some strains could be regrown after 24 months. The main advantage of this method is the avoidance of the need to use specific subculturing preservation techniques for each plant

tissue: one set of conditions and protocols can be widely applied. Augereau, J. M., Courtois, D. and Petiard, V. (1986) Plant Cell Rep. 5, 372-376

Soybean protoplast culture Protoplast isolation and culture are proved particularly difficult but a new necessary steps in the controlled genetic technique has now been described. Protoplasts of soybean cells were manipulation and regeneration of plant species. Soybean protoplast culture has encapsulated in calcium alginate beads.

Industrial waste water treatment: the anaerobic alternative Paul Ditchfield

This article describes the rationale behind the biological treatment of industrial effluents, specifically waste waters containing organic pollutants, and discusses the two major options available, aerobic and anaerobic systems. The article defines the major differences between the two methods and considers the advantages, both economic and biological, of the anaerobic system. Manufacturing processes often result in the production of waste water streams containing both soluble and insoluble chemical and biochemical oxygen demand. If this effluent were to be discharged directly into a water course, rapid deoxygenation of that water course would result, with serious consequences for the natural fauna and flora. Therefore, the producers of waste waters either have limits imposed upon them concerning the quantity and quality of effluent which can be discharged or, alternatively, channel waste via the sewer to a water authority P. Ditchfield is at Biomechanics Limited, Caxton House, Wellesley Road, Ashford, Kent TN24 SET, UK.

treatment works where the pollutants are removed. Not unnaturally, the authority levies a charge for such treatment and, in the UK, this is based on the Mogden formula (see below). Indeed, in many cases the authority will insist on effluent being partially or fully treated on site, prior to accepting it for treatment and disposal. This is particularly so in areas where the overall volume of domestic and industrial effluent is greater than can be effectively treated by the local sewage works. The treatment and subsequent disposal of industrial effluents is therefore of great concern to processing industries. On the one hand there is a financial burden, on the

Growth was induced by culturing the beads in a modified Gamborg's B-5 medium which had been inoculated with 10% v/v of actively growing soybean cells. These feeder cells were essential to cell colony development from the protoplasts. Protoplasts could divide and form callus colonies even at very low densities. Colonies derived from single protoplasts could, therefore, be cultured. Tricoli, D. M., Hein, M. B. and Carnes, M. G. (1986) Plant Cell Rep. 5, 334-337

other legislative and environmental pressure. To overcome these mounting problems, many industries - food processing, chemicals, brewing and distilling, textiles and paper making - have turned to on-site effluent treatment.

Anaerobic versus aerobic There are two basic modes of operation in on-site processes for the biodegradation of organic matter, namely aerobic and anaerobic. Aerobic biodegradation involves the bacterial conversion of waste organic matter to new bacterial cell matter which can subsequently be dewatered and disposed to land. In contrast, the anaerobic process converts the pollutants via a series of metabolic reactions to gaseous end-products, specifically carbon dioxide and methane, the latter being a combustible by-product. Essentially, the difference between the two metabolic pathways is that one produces principally solid end products, the other gases. There are therefore savings associated with the anaerobic option, from reduction in sludge disposal costs and the net gas benefit. In addition, while aerobic treatment requires an oxygen supply and relatively high nutrient levels (mainly nitrogen and phosphorus), anaerobic systems function in the absence of oxygen and can sometimes operate without added nitrogen and phosphorus. The two systems are illustrated in Fig. 1.

Anaerobic metabolism Anaerobic treatment of industrial waste water involves a wide variety of symbiotic microorganisms. These

© t986, Elsevier Science Publishers B.V., Amsterdam

0166- 9430/86/$02.00

b

TIBTECH

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form complex associations, the combined action of which is to metabolize the polluting organic material, ultimately to carbon dioxide and methane. There are three major groups of bacteria involved in the process. The hydrolytic and acidogenic microorganisms are facultative and obligate anaerobic bacteria. The methanogenic organisms are strict (obli-

gate) anaerobes and are inhibited by the presence of oxygen; they are only distantly related to other bacteria and are characterized by the cellular presence of unique nickel-containing cofactors and lack of sensitivity towards antibiotics. The three types of bacteria depend on each other for the supply of appropriate nutrient substrates and maintenance of a suitable environ-

- Fig. 1

Anaerobic No odor I-lncnf-4

fmm

&

Energy output L (methane

gas)

treated effluent

Raw Effluent to be treated

I

Small amount ) of stable sludge

energy I- Small inDut for gentle

mixing

Aerobic

I

Raw effluent to be treated I ,

A comparison

of anaerobic

Activated Sludge Process

’

95% treated effluent

i Major problem to _ dispose of large ’ Large energy quantities of I input for unstable sludge iniense mixing and aeration and

aerobic

processes.

ment (e.g. the correct redox potential, ionic balance and, in particular, an extremely low hydrogen ion concentration). The population of microorganisms, when used in a treatment plant, is known as the biomass or anaerobic sludge. Figure z outlines the anaerobic process at the molecular level. It also illustrates the close relationship and interdependence between the various bacterial groups to be found within an anaerobic digestor. Macromolecules [carbohydrates, lipids and proteins) are broken down by a series of liquefying and hydrolytic steps to smaller soluble molecules, which the acid-forming bacteria use for the formation of acetic acid or, alternatively, higher molecular weight fatty acids (butyric and propionic). The acetogenic (acetate forming] bacteria then convert the higher fatty acids to acetic acid. The metabolic process has thus produced the three major substrates from which the methanogenic bacteria can produce methane: acetic acid, hydrogen and COZ. There are two major groupings within the methanogens, namely the acetoclastic organisms which produce methane by the decarboxylation of acetate and the hydrogen-utilizing bacteria which reduce carbon dioxide with hydrogen for the production of methane. Approximately 70% of the methane produced in an anaerobic digestor arises from the acetoclastic pathway, but the importance of the Hz-utilizing methanogens should not be underestimated: they remove hydrogen from the system (see Fig. 2) and this removal promotes the conversion of propionic and butyric acids to acetic. A build up of these higher acids is thereby averted and the digestion process proceeds under stable, steady state conditions. Substrateoverload Of the groups of organisms within the system, the methanogens have the slowest growth rates and are the most susceptible to variations in environment conditions (e.g. in pH). If there was a sudden influx of feed (waste), the hydrolytic and acidforming bacteria would metabolize it, leading to a rapid increase in digestor acetate, butyrate and propionate

TIBTECH - DECEMBER 1986 ~-

Fig. 2 PROTEINS

LIPIDS

(Hydrolysis) Peptides

CARBOHYDRATES

(Hydrolysis)

(Hydrolysis)

Glycerol + long chain acids

I

Amino acids

Hexoses + Pentoses

(i6-Oxidation)

(Deamination)

ACID FORMING BACTERIA ,q~

H2

. Pyruvic acid

- - Acetic acid

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Butyric acid

I

I ACETOGENIC BACTERIA

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Acetate decarboxylation (Acetoclastic methanogens)

C Q reduction (H2-utilizing methanogens) '

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CH4 + CO 2

An outline of anaerobic digestion at the molecular level.

levels; the more slowly responding methanogens would temporarily be unable to metabolize the increased level of their substrate. Consequently, concentrations of the intermediate acids would rise, causing the digestor liquor pH to drop. This in turn would inhibit the methanogens, and butyric and propionic acids would build up because hydrogen would not be removed from the system. The initial cause of an interruption of methanogenic metabolism could be a reduction in pH with a subsequent decrease in the degree of ionization of fatty acids, rather than an increase in levels of volatile acids within a digestor. In a well run, fully equilibrated

digestor, however, the whole m e t a bolic pathway proceeds efficiently with low volatile acid concentrations. If overloads or pH imbalances occur, pH correction will ensure that no inhibition of the methanogenic organisms will result. Such safeguards are easy to install in any anaerobic system.

Anaerobic treatment plant technology and design Figure 3 illustrates the main components of an anaerobic treatment unit. Waste water is fed to the digestor, which contains the necessary quantity of biomass. The digestor can be a closed tank which is kept fully mixed by gentle stirring:

this is often referred to as the contact process. More complex reactor configurations such as packed bed and sludge blanket are used under certain circumstances. With some wastes it is beneficial to separate the two major activities within the anaerobic system: (1) the formation of intermediate volatile acids, and (2) the formation of methane. In such a two-phase system the organic compounds from waste would be converted to intermediates in the acidogenic reactor with only negligible reduction of COD (chemical oxygen demand) and negligible gas production; in the second methanogenic reactor these intermediate metabolites would be converted to methane and carbon dioxide, thus delivering the required overall COD reduction and associated gas benefits. The low production of new biomass, while an advantage to the user of the process, creates a challenging technological problem for the plant designer: the plant design must conserve the biomass by ensuring that the slight loss of bacteria in the effluent from the plant does not exceed the production. A number of options have been researched, developed and tested to retain biomass within the reactor: ultrafiltration, simple sedimentation preceded by de-gassing or thermal shock, attached-biomass systems using fluid or static beds, granular biomass and laminar flow sedimentation are some of the more established approaches.

Effluent characterization and treatability studies Each effluent stream will exhibit its own specific profile of chemical and biological oxygen demand, solids concentrations, pH, and concentrations of anions and cations. Effluent analysis is therefore essential in obtaining an initial assessment of the potential of the anaerobic process to degrade the effluent organics and allowing a suitable experimental design to be constructed. It is important to establish the presence and concentration, if any, of essential nutrient elements. Screen.ing for possible toxic constituents in the waste is also carried out at this stage. Once this preliminary character-

TIBTECH - DECEMBER 1986

ization of the effluent has been accomplished, consideration can be given to the choice of hardware for the effluent treatability study. Preconditioning of the waste (e.g. pH adjustment, screening of macrosolids, dilution, thermal optimization, nutrient supplementation and removal of recalcitrant toxic species) also forms an integral part of the background to the study. It is then possible to test the ability of the chosen anaerobic package (biomass and conditions) to degrade the waste organic compounds of the effluent using a laboratory scale (reactor volume 5-10 litres) to pilot plant scale {6-20 m 3) digestion unit. The most important parameters to monitor during the treatability study are the carbon and solids mass balances across the system, in order to determine COD and solids removal rates. One would also wish to identify optimum organic loading rates, assess acclimatization periods, determine any undefined nutrient requirements, and investigate process stability under steady state conditions. The effects of any pre-adjustment of waste characteristics, suppression of the effects of toxic compounds and possible refinement options would also be noted. The treatability study would, therefore, generate a data base upon which economic and engineer-

ing considerations for a full scale plant could be based.

mentation had identified both a suspended growth and a packed bed reactor to be equally effective in degrading the organic substrate. However, the client has specified that the COD of the effluent must be reduced by at least 90%. The high rate of throughput in the packed bed reactor means that it is possible to consider a smaller unit for treatment of the same volume of effluent. However, the COD reduction in the packed bed reactor may not always be sufficient to meet the client's specification. Therefore, unless physical site constraints (e.g. ground loading or space) have a significant impact on the economics of the plant, the suspended growth system, which can be guaranteed to achieve the objective, will be preferable. Where space is limited, the implementation of engineered (imposed) process stability in the reactor may be necessary. Within smaller reactors, hydraulic retention times will be lower, and the effect of disturbances, such as shock overloads and pH falls, will be more marked. Engineered process stability, for example adaptive biosensor control, would preclude the need to design in excess capacity, and is a far less vulnerable method of protecting against process disturbances. High effluent solids concentrations

Practical design considerations There are several reactor designs which accommodate the necessary anaerobic microbiological activity. All have been used in larger scale applications but each has advantages and disadvantages to the specific process user. Whilst increasingly higher loading rates can be achieved under strictly defined scientific conditions, in practice the key design objective is to apply the technology to ensure the maximum benefit to the end user. Operational simplicity, for instance, facilitates effective management of the digestion system and ranks high in the criteria of hardware design. Process reliability and robustness are qualities near the top of any prospective end-user's specification. Operators require systems which are designed to exclude t h e greatest number of reasons for substandard performance and certainly do not want facilities which are run at their extreme limits and which would require the constant attention of key personnel and complicated esoteric control mechanisms.

Design examples Let us consider a hypothetical example in which the initial experi-

-- Fig. 3

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TIBTECH - DECEMBER 1986 - Table 1

Effluent treatment charges levied per annum by local authorities aand savings made by on-site anaerobic effluent treatment b Water authority

Brewery

Paper mill

Charge levied (£)

Saving (£)

Charge levied (£)

Saving (£)

310170 241 180 465100 360 150 567470 485150 332 850 368 550 353310 247950

220130 180 330 353 060 268010 425670 365 610 253 260 291 000 260820 174610

147 250 114150 219 615 168 100 269115 225 790 160 350 170 020 156650 121 870

96430 80 520 159 225 117 220 192 130 160 280 118140 129160 117 150 81 080

Anglia North West Northumbria Severn Trent South West Southern Thames Welsh Wessex Yorkshire

aThe treatment charges were calculated using the Mogden formula and the following assumptions. The brewery: 1500 m 3 of effluent per day with an average pollutant load of 3000 mg I-1COD and 650 mg 1-1 suspended solids. The paper making plant: 1000 m 3 of effluent per day with an average pollutant load of 2000 mg 1-1 COD and 600 mg 1-1 suspended solids. In each case a 350 day working year was assumed. bin calculating the saving in levied charges, it was assumed that COD could be reduced by 90% and suspended solids reduced by 75% by on-site pretreatment.

or the presence of immiscible lipid components which require biodegradation may necessitate treatment by a fermentor having a longer solids retention time and fully mixed digestor contents. This will ensure sufficient contact between poorly soluble substrate components and microorganisms. Naturally, all wastes have different characteristics and users of anaerobic processes each have a different hierarchy of preferences imposed on The Mogden formula

C=R+V+

Ot B + l s s I

C = Total charge per cubic metre of trade effluent (pence) R = Reception charge (pence) V = Primarytreatmentcharge (pence) B = Biologicaltreatment charge (pence) S = Sludge treatment and disposal charge (pence) Ot = COD(mg 1-1) of the trade effluent after one hour quiescent settlement at pH7 Os = COD (mg 1-1) of settled crude sewage St = Total suspended solids (rag 1-1) ofthetrade effluent at pH7 Ss = Total suspended solids (mg I-I) of crude sewage

them in their choice of plant design. The major impositions are obviously economic, legislative and sitespecific criteria. The final choice of anaerobic treatment plant should be the best balance and compromise between all these factors, so that the operator derives the maximum benefit in both biological and commercial terms. Financial considerations Earlier in this article, reference was made to the Mogden formula. This is used by all UK water authorities to determine the cost of effluent treatment (see Box). With the exception of Ot and St, all components of the formula are fixed by each water authority. Table 1 illustrates for a brewery and a paper mill the typical treatment costs levied by various water authorities, as determined using the Mogden formula. Treatment costs vary because these fixed figures are different for each water authority. For instance, Wessex charges 2.0 pence for R, while Northumbria charges 7.75 pence. Table I also shows the saving that can be made by on-site pretreatment of trade effluent. The figures used in calculating this saving (90% reduction in COD, 75% reduction in suspended solids) can be easily met in practice by anaerobic processes. In addition to reduced effluent

treatment charges levied by the water authority, very significant cost savings can also be derived from methane gas generation if an anaerobic digestor is used. Typically, 0.350.45 m 3 of biogas is produced per kilogram of COD applied to the anaerobic digestor. Digestor gas comprises, on average, 75% methane and has a calorific value of approximately 23 MJ per cubic metre, equivalent to about 21427 BTUs. Therefore if we turn to the brewery example again, the installation of an anaerobic digestor would provide further fuel savings of between £35000 and £48000 per year (gas equivalent value). This, together with reduced treatment costs levied by the water authority, means that pay-back periods on capital plant expenditure could be as little as one year. Future outlook Anaerobic effluent treatment technology is now well established as a viable alternative to traditional aerobic processes. Scientific and technological advances over the last 15 years have laid the foundations for its use within a diverse range of industries. Effluents from food processing and related industries - dairies, farming, edible solids, breweries and distilleries - have all been treated successfully by anaerobic methods, as have (more recently) effluents from the chemical, textile and pharmaceutical industries. Indeed, this area, the treatment of 'difficult' effluents, is one of the greatest challenges confronting the effluent treatment industry today. The removal of 'colour' from waste water and the treatment of particularly toxic chemical effluents are just two instances in which anaerobic treatment has been shown to succeed. It is clear that the environmental and financial benefits to be gained by opting for the anaerobic alternative will grow in line with the continuing escalation of water treatment charges levied by the regional water authorities and a growing public awareness regarding waterborne pollution and its effect on the environment.