Anaerobic-aerobic biotreatment of domestic sewage by LEVAPOR biofilm technology

Leverkusen, July 2011.

1. INTRODUCTION The worldwide most practicized method for the treatment of domestic sewage represents their biotreatment under aerobic conditions ( fig. 1), achieving a remar-kable removal of main organic and inorganic pollutants, however this process has also two remarkable problems:

Fig.1 Basic flow diagram of a classical aerobic biotreatment plant

High energy consumption for aeration of the liquid phase with suspended activated sludge (ca. 0,5 to 0,8 kWh/kg O2-input), representing the highest cost block of a plant and High excess sludge production of aerobic processes, which must be treated in a subsequent step approximately at similar cost level like sewage treatment. During the biotreatment process, about 40 % of removed COD and 80% of BOD become converted into low degradable excess sludge, comprising mainly of microbial cells, digested often under anaerobic conditions, where due to the weakly hydrolysable microbial cell walls only 25 to 35 % of the biomass can be converted to biogas, while the residual mass must be conditioned and incinerated, respectively dumped in landfills. Anaerobic biotreatment of effluents represents a nearby energy-autarch alternative method, comprising of acidification and subsequent methanisation of organic pollutants, generating remarkable amounts of energy rich biogas under much lower sludge production. Due to their low COD concentrations and low temperatures during winter periods, this method has not yet been established in sewage treatment . With regard to recent developments on energy sector, anaerobic-aerobic treatment of domestic sewages becomes a very attractive alternative, especially for geographic ares with elevated average temperatures, like Mediterranian area, Middle-East and numerous Asian respective American countries. Biofilm technologies using innovative LEVAPOR carriers, highly adsorbing porous polyurethane cubes are established since 20 years in aerobic and anaerobic biotreat-ment of wastewater and polluted gases, their application in anaerobic treatment of municipal sewage however, has been tested successful in pilot scale.

Aerated basin Clarifyer

2. PROCESS AND PLANT PROPOSAL For increasing of plant efficiency and remarkable reduction of treatment costs, anaerobic sewage pre-treatment in biofilm reactors, followed by aerobic post treatment in existing aerated basins is recommended.

Fig.2 Basic process flow diagram of anaerobic-aerobic effluent treatment od municipal effluents by biofilm technology

2.1. PROCESS DESCRIPTION After a mechanical pre-treatment by a grid and aerated sand trap, raw sewage arrives into the buffer with variable water levels, where additionally to hydraulic equalisation a hydrolysis of fine dispersed pollutants and first biochemical conversion of organic pollutants into volatile fatty acids takes place. From here a constant quantity of acidified sewage is pumped into the anaerobic reactor, containing active methano-genic bacterial biofilms, fixed on adsorbing porous LEVAPOR-carrier , enabling a fast and quantitative microbial colonisation and retention of active cells in the reactor. In this reactor bioconversion of generated acetic acid into an energy-rich mixture of ca. 2/3 methane and 1/3 carbon dioxide, called biogas, takes place at ambiental temperatures. The sludge growth in anaerobic process is only 3 to 5% of removed COD, instead of 35% in the aerobic treatment, resulting in a remarkably better process economy. Final treatment of residual pollutants occurs in a subsequent aerobic fluidi-sed bed biofilm reactor, where additionally to COD removal, quantitative nitrification of ammoniacal nitrogene into nitrate (NO3

-) takes place. Aerobically treated effluent, containing suspended biomass leaves the reactor to the clarifier, where sludge flocs will settle, while retention of LEVAPOR cubes with fixed biofilm occurs by a screen, installed inside of the reactor. The obtained biogas can be utilized in a gas-powered combined heat and power unit for generation of electricity and heat, improving remarkably the process economy and reducing the CO2-emissions .

Anaerobic reactor

Aerobic reactor

Excess sludge

Biogas

Buffer

3. PLANT DESIGN 3.1. Wastewater data for 150.000 population equivalents (P.E.):

1. Average water flow, Q = ca. 15.000 m³/d = 625 m³/h 2. COD = 18.000 kg/d 3. BOD = 9.000 kg/d 4. TKN = 1.800 kg/d

3.2. Plant design and basic data

a) Equalisation / buffer , VE = 3.000 m³ b) Anaerobic reactor, VANA = 9.000 m³ ( 2 * 4.500 m³) c) LEVAPOR carrier VLVP= 1080 to 1.350 m³ (12 to 15 vol.%) d) Aerobic reactor VAE= 2500 m³ e) O2-input GO2 = 7.500 kg/d

4. ESTIMATED PLANT PERFORMANCE In our tests carried out at room temperature, anaerobic LEVAPOR biofilm reactors achieved 65 to 75% COD removal, followed by further COD removal and quantitative nitrification in a subsequent aerobic biofilm reactor. In a pilot plant study of Deutsche Bundesstiftung Umwelt, (Project nr. 10.144),carried out in anaerobic-aerobic and denitrifying fixed bed reactors,

30 to 70 % COD-removal and

70 % N-removal were achieved, under

70% lower energy costs and

67% lower degree of sludge production. Using LEVAPOR-System, COD-removal rates of 92-96 % and N-removals of 75 to 90% under 75% lower sludge production can be expected. PROCESS ECONOMY OF THE PROPOSED TECHNOLOGY

Legend Dimens. Existing aerobic process

Proposed ANA-AER-process

Difference

O2-input Kg/d 22.500 7.000 - 69,9 %

Energy consum. Kwh/d 11.300 3.500 - 69,0 %

Biogas m³/d -- 5.985

Energy yield at 6,5 kWh/m³ -- 38.903 + 38.903

Excess sludge Kg/d 9.870 2.893 - 70,7 %

Table 1. Expected results of the anaerobic-aerobic plant

6. ADVANTAGES OF THE PROPOSED LEVAPOR TECHNOLOGY As mentioned in previous section, in a similar pilot project remarkable savings had been achieved. Due to the higher surface of LEVAPOR carrier and better mixing in the planned fluidised reactor, in this project a system with

Smaller reactor size

Higher bioconversion rate

Higher biogas production

Better process economy via biogas utilisation

Higher process stability will be realised and

High removal of hazardous micropollutants and endocrine disruptors

Eventually required nitrogene removal could be achieved in existing reactors. Due to the estimated realistic advantages, the proposed technology could become a remarkable efficient alternative for the sewage treatment in warmer climatic areas.

Our additional services we do offer also our services in designing taylor made problem solutions, based on 40 years experiences on biofilm technologies both in science and in the practice. Our tools are:

Analysis of the problem

Elaboration of alternatives for problem solution , supported by

Practice oriented biotests (optional),

Process Design and/or Engineering

Production and delivery of the required LEVAPOR type and

Plant startup using optimized mixed biomass, enriched with microbes essential for degradation.

Presented informations are based on experiences with application LEVAPOR carrier. Testimonies on expected

effects can be made in individual case only on basis of investigations of given emissions and in some cases on

basis of practice relevant experiments.

LEVAPOR GmbH

Kölner Str. 38 Tel.: + 49- 2173-938715 D- 51379-Leverkusen Mobile: + 49- 177-786 5533

Germany www.levapor.com E-mail: levapor@web.de