anaerobic digestion for wastewater treatment in key words—anaerobic digestion, wastewater,...

ANAEROBIC DIGESTION FOR WASTEWATER

TREATMENT IN MEXICO: STATE OF THE TECHNOLOGY

OSCAR MONROY*, GRACIELA FAMAÂ , MOÂ NICA MERAZ,

LETICIA MONTOYA and HERVEÂ MACARIE{M

Department of Biotechnology, Universidad Auto noma Metropolitana-Iztapalapa, Apdo. Postal 55-535, Me xico, D.F. 09340 Mexico

(First received 1 July 1998; accepted in revised form 1 July 1999)

AbstractÐDue to the nascent wastewater treatment practice in Mexico there is great opportunity to introduce anaerobic digestion as the core of the wastewater treatment processes. Nevertheless, this requires an understanding of all the technical, economical and ®nancial aspects that limit its development. According to the National Water Commission, in 1995 municipal and industrial wastewater were produced at rates of 232 and 168 m3 sÿ1 respectively, but only 20 and 12% of these volumes were treated, often with very low eciencies. In order to increase the treatment capacity of the country, approximately US$ 4515 million managed by the banks for development are supposed to be available to invest in environmental projects. Other ®nancing mechanisms exist through treasury incentives and penalties. Within this situation, anaerobic digestion has grown although not at the required rate and bigger investments are being made on conventional aerobic and physicochemical technologies. Presently there are in the country 85 anaerobic wastewater treatment plants treating 216,295 m3 dÿ1 with an installed volume of 228,551 m3. UASB reactors account for 74% of the installed volume and national companies have supplied 76% to the anaerobic market. Proper integration of the anaerobic digestion processes for water recycling and energy recovery has not been achieved and there is a big need to demonstrate economic and ecological sustainability. # 2000 Elsevier Science Ltd. All rights reserved

Key wordsÐanaerobic digestion, wastewater, Mexico, technology, industrial, domestic, municipal, bio-

gas use, treated water reuse

INTRODUCTION

There is a great public concern in Mexico for the

origin and fate of water and wastewater. Due to this concern and to the recent environmental laws, there has been a great investment in municipal

water supply and wastewater collection and treat- ment (see Table 1).

Municipal wastewater

Since 1988 the growth rate of municipal waste- water treatment plants (WWTP) has been of 102 plants or 4.8 m3 sÿ1 per year (Table 2). Despite this relative high growth rate (compared to the econ- omic growth), the gap between the treated 47 m3

sÿ1 and the produced 232 m3 sÿ1 of sewage is still very large (Table 1), giving place for advanced and inexpensive technologies. A closer analysis of the small fraction of treated

wastewater will show that out of the 946 municipal

treatment plants, approximately 40% are stabiliz-

ation ponds. The second largest number corre-

sponds to activated sludge plants which together

with oxidation ditches, aerated ponds and trickling

®lters, make up another 40% of the treatment sys-

tems. From these plants, only 755 (79%) are in op-

eration, 41% (312 plants) have BOD removal

eciencies higher than 75 and 26% (199 plants)

have an eciency lower than 50%. This is because

the treatment systems are of dierent types as

shown in Fig. 1. Average capacity of these plants is

42 L sÿ1 ranging from 5000 to 1 L sÿ1. The front columns in Fig. 1 show the distribution of the

plants which are not in operation. It can be seen

that, probably due to overloading conditions, most

of them are stabilization ponds (9% of total) and

primary treatment systems (3% of total). Aerobic

processes account for 30% of the nonoperating fa-

cilities due to lack of aerator's maintenance.

Moreover, very few of them have sludge treatment

facilities, which added to the high operating and

investment costs, make them a nonviable option in

the long term.

Wat. Res. Vol. 34, No. 6, pp. 1803±1816, 2000 # 2000 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0043-1354/00/$ - see front matter

1803

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PII: S0043-1354(99)00301-2

*Author to whom all correspondence should be addressed; e-mail: monroy@xanum.uam.mx

{Invited researcher from Institut de Recherche pour le Developpement (IRD), France.

Industrial wastewater

According to the National Commission for

Water (CNA), by 1994, industrial wastewater was produced at a rate of 168 m3 sÿ1, 12% being treated in 282 treatment plants, 61% released untreated to

the environment and 27% discharged to sewers. The sugar cane industry generated 39% of this volume, 21% the chemical industry, 22% the paper, petrochemical and oil industries and 18% was pro-

duced by other industries.

LEGAL, ECONOMIC AND FINANCIAL ASPECTS

In 1988 the Mexican government issued the

General Law for the Ecological Equilibrium and

Environmental Protection which triggered an

intense activity to match the Mexican industry dis-

charge standards to those of their partners in the

NAFTA (North America Free Trade Agreement).

This activity is characterized by: (a) the continuous

inspection of industrial discharges with consequent

partial or total closures, (b) penalization for the

amount of wastes discharged (CNA, 1993), (c) an

advertised fund availability for any kind of pol-

lution control facilities, (d) hundred of studies for

the preparation of Ecological Guidelines (INE,

1993±1994) and programs for human resources

training (Jime nez, 1995).

Table 1. Drinking water and sewage nets, coverage and growth rate in 1995. MWW=municipal wastewatera

Flow rate (m3 sÿ1) Population covered % Growth rate (% per year)

Drinking water net 272 86.2 4.34 (population based) Total MWW produced 232 ± ± MWW in sewers 120 69 8.47 (population based) MWW treatment 47.6 14.5 14 (plants based)

aSource: CNA (1995±1996).

Table 2. Growth pattern of municipal wastewater treatment plants. n.d.: non determineda

Year No. of plants Treated ¯ow rate (m3 sÿ1) Expected removal (103 kg BOD5 d ÿ1)

1988 233 14.0 302 1989 256 15.2 343 1990 310 19.3 418 1991 361 25.1 541 1992 577 29.1 627 1993 650 34.8 750 1994 825 38.4 830 1995 946 47.6 n.d.

aExtracted from Sancho (1992) and CNA (1995±1996).

Fig. 1. Operating and nonoperating municipal wastewater treatment plant distribution per type. SP=stabilization ponds, AS=activated sludge, PT=primary treatment, BF=trickling ®lters, AP=aerated ponds, OD=oxidation ditches, IT=Inmho tank, ZZ=others (biological disks, counter-

¯ow aeration, lemna pond), from CNA (1995±1996); MejõÂ a (1993).

Oscar Monroy et al.1804

The Environmental Budget grew from US$ 6.6

million in 1989 to US$ 78 million in 1992 (Cero n,

1993). Currently, according to the Secretariat of

Ecology (Semarnap), from 1995 to 2000, there will

be an investment in the environmental market of

US$ 4515 million (La Jornada, 1996). These funds

will come from banks for development and from

national and foreign private investments. To our

knowledge these banks do not provide readily avail-

able fresh funds for most of the middle and large

sized industries. For this reason, industries tend to

invest on their own pro®t expense rather than bor-

rowing from banks. Their selnancing has been

propelled by charges on COD, SS and wastewater

volume discharged as well as the cost on tap water.

Discharge limits should be gradually reduced all

over the country during the next twelve years to

reach a 20:20 (BOD5:SS) quality for water reuse, as

stated by the guideline NOM-003-ECOL-1997,

recently edicted. NOM-001-ECOL-1996 establishes

30:40 as the maximum permissible limits for pollu-

tants in wastes discharged into super®cial waters.

The guideline NOM-002-ECOL-1996, does not

establish limits for BOD5:SS in euents discharged

into urban or municipal sewage systems. These

guidelines take into account the producer's socioe-

conomic level, infrastructure and population size.

Euents should adequate to particular discharge

limits depending on the discharge site's dilution ca-

pacity and use of water. Treatment processes to

meet the discharge consents are left to the user's

choice.

Cities with larger treatment facilities, turn out to

contract the services from foreign or national pri- vate companies which invest under service contracts to build and operate new or existing facilities under

a BOT (build, operate and transfer) scheme, while the government faces the responsibility with the public. Contracts can include a total privatization

of the facilities or its recovery by the municipality upon an agreed number of years. This system pro- vides the cities with wastewater treatment plants at

no initial cost, nevertheless these kind of contracts have been, thus far, dicult to negotiate due to the risks involved with the long term operation of plants.

DEVELOPMENT OF ANAEROBIC DIGESTION IN MEXICO

The use of anaerobic digestion (AD) for waste- water treatment started late in Mexico compared to the European countries or even to North America. The ®rst digester was constructed by 1987 (see Fig.

2). Fu