Decomposition System ofNitrogen Compounds in Waste
Water with Electrolysis
MASAMICHI ASANO* 1
KENJI NAKAMURA* 2
YUUDAI KATOU* 2

HIROSHI MIZUTANI* 2
TAKASHI IKE* 2

With waste water contained nitrogen compounds becoming a growing problem, legal controls are being promoted
to treat nitrogen compounds. Conventional biotreatment for removing nitrogen requires much space for installing
conventional large-scale facilities. Electrolytic denitrification completely unlike conventional biotreatment and using electrolysis has been developed and commercialized to solve this problem, as detailed below.

1. Introduction

2. Comparison of nitrogen removal technologies

Conventional biotreatment for removing nitrogen is
widely applied to different types of wastewater, turning
nitrogen into harmless nitrogen gas. Biotreatment, whichis easily affected by load variation, requires complex control depending on the load status to keep appropriate
activity of microorganism and addition of methanol to accelerate denitrification from wastewater with a low
carbon/nitrogen (C/N) ratio. The resulting sludge treatment is also required. Cost reduction and easy operation
are becoming big problem in waste water treatment. To
improve the facility of nitrogen removal, large-scale construction such as installing new equipment is required,and makes it difficult to implement measures for preventing nitrogen emission via biotreatment.
Given this background, Mitsubishi Heavy Industries,Ltd., (MHI) has developed denitrification technology differing from conventional biotreatment and using
electrolysis, commercializing electrolytic denitrification
applicable to wastewater containing a wide range of nitrogen concentrations and to organic wastewatercontaining organic substances in hybrid use with theconventional biotreatment.

Table 1 shows nitrogen removal technologies developed to date. Different nitrogen removal technologies
have been put to practical use, but compared tobiotreatment, few have been actually applied.

3. Electrolytic denitrification features

Ammonia stripping

Effective forhigh-concentration
wastewater

Removal of ammonia
alone possible, hightreatment cost

Ion exchange

Effective forlow-concentration
wastewaterHigh construction andtreatment cost, requiring
waste solution
reprocessing

Electrolytic denitrification features the following:(1) Stable treatment
Stable denitrification is ensured because electrolytic denitrification is stable in removal using
electrolysis, and responds immediately to load variations by adjusting the electrolytic current and chloride
ion concentration.
(2) Easy operation
Compared to conventional biotreatment, electrolytic denitrification handles load variations,temperature changes, etc., of treatment solution flexibly, ensuring stable nitrogen removal and reducing
power and labor for operation. Because equipment iseasy to operate, it enables continuous, automatic operation.
(3) Space-saving
Being compact, equipment requires minimal spaceand no large-scale remodeling at installation, dramatically reducing construction time.
Electrolytic denitrification are used independently
and automatically adjust treatment capacity, facilitating their application to existing treatment
facilities.
(4) Reduction of secondary waste
Unlike biotreatment, electrolytic denitrification removes nitrogen from wastewater with a low C/N ratioor containing little organic matter without adding
carbon sources such as methanol, etc. This produces
less sludge than biotreatment, dramatically reducing
secondary waste.

*1 Yokohama Research & Development Center, Technical Headquarters*2 Yokohama Dockyard & Machinery Works

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 4 (Nov. 2005)

Table 1 Denitrification technologies
Denitrification
technologies

Advantages

Problems

Biotreatment(Nitrification anddenitrification)

Harmless conversion
to nitrogen gas, wide
application range

Requires large installation space and longtreatment time.

Chlorine injection at Low construction cost High treatment cost anddiscontinuous points and easy application probable byproducts

1(1) Problems with night soil treatment equipment
Recent nationwide trends in diluting night soil topromote flush systems and the spread of combination
septic tank have generated large amounts of septictank sludge and substantial variations in waste properties, making it difficult to maintain stable
denitrification by basic biotreatment equipment,
which cannot be managed only by operation control
efforts. Intensive stable denitrification is thus needed.
Electrolytic denitrification require that treated solutions have a suspended substance (SS) content of
1 000 mg/L or less. Electrolytic denitrification areapplicable in night soil treatment to solutions for dehydration and separation before the generation of
septic tank sludge with low SS content and ammonia-nitrogen or to solutions for solid-liquid separation
after biotreatment. Hybrid treatment combining conventional biotreatment with the electrolytic
denitrification using electrolysis for denitrification
have been developed and commercialized.
(2) Features of hybrid electrolytic denitrification
Table 2 compares conventional biotreatment technology. Compared to conventional biotreatment
denitrification, hybrid treatment entails shorter construction time and ensures more effective
denitrification at lower remodeling cost.
(3) Basic electrolytic denitrification configuration
Electrolytic denitrification consist of a power supply including a control panel and DC power supply,
an electrolytic tank, pumps, and exhaust fans.
The unit is operated basically using batch treatment involving feeding water from raw water tanks
and water treatment tanks preceding and following
the electrolytic tank , electrolysis and discharge transferring treated solutions to the next process, andFig. 2
repetition of these processes (Fig.
2).

4. Electrolytic denitrification reaction mechanism
The denitrification mechanism in an electrolytic denitrification, shown below, implements excellent removal,
Fig. 1
especially of ammonium-nitrogen (Fig.
1).
(1) The electrode reaction at the anode generates chlorine (Cl 2 ) from chloride ions (Cl - ) (equation 1),hydrolyzing chlorine and generating hypochlorousacid and hydrochloric acid (equation 2).
(1)
2Cl- ----> Cl2 + 2e(2)Cl2 + H 2 O ----> HClO + HCl(2) Ammonia in wastewater reacts with hypochlorous acidgenerated in equation 2 to produce monochloroamineand dichloroamine (equations 3 and 4).
(3)NH3 + HClO ----> NH 2Cl + H2O(4)NH2 Cl + HClO ----> NHCl2 + H2O(3) Monochloroamine and dichloroamine react, becoming harmless nitrogen gas.
(5)
NH2 Cl + NHCl2 ----> N2 + 3HCl

5. Application to night soil treatment
MHI's new system is applicable to inorganic and organic wastewater containing ammonia at widelyconcentrations. Application to night soil treatment involves the following:

N2AnodeCathodeH2-Cle-Cl2e-Table 2 Comparison of biotreatment

HCIO

NH3

Conventional
denitrification(biotreatment)

Hybrid electrolyticdenitrification

Biologicaldenitrification

Electrolytic denitrification+ biotreatment

Long time

Short time

Large

Small

4. Operability

Complex control,
long treatment time,and poor flexibility

Correspondence to concentration variation throughautomatic control, showing
capacity immediately afteroperation

5. Maintenanceperformance-Easy maintenance becauseof simple equipment

6. Construction
timeLong, requiringconstruction work

Short

1. Treatment
overview
2. Treatment
time
3. Installation
space

Fig. 1 Principle of electrolytic denitrification

Chlorine ion

Deodorization process
B Exhaust blowerFeed pump
PRawwaterRawwatertank

Waste pump
Electrolytictank

P

Treatedwater

Treatedwatertank

7. Operating
costFig. 2 Overview of electrolytic denitrification

Almost equivalent(Depending on treatment conditions)

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 4 (Nov. 2005)

2

Planned electrolysis time t A per batch is fixed fromthe planned volume of treatment per day and effective
electrolysis time t S obtained by subtracting time t I for
solution supply and time tO required for discharge fromplanned electrolysis time tO: tS = t A - tI - tO.
If denitrification ends within effective electrolysis
time ts (case A in Fig. 44), current in the next batch isdecreased. If denitrification does not end within t S(case B in Fig. 4), current in the next batch is increased. Optimized control thus realizes automatic
optimization of current, ensuring treatment using onlyrequired electrolysis time without excess electrolysis.
(5) Delivery record(a) Background
The first electrolytic denitrification for removing
nitrogen from night soil started in October 2004. The
system continues to run smoothly, stabilizing denitrification.
(b) Applicable flow
The applicable flow and example of electrolytic
denitrification at the human-waste treatment facility are shown in Figs. 5 and 6 . The flow is applied toa membrane filtrated solution after biotreatment.

(4) Control under optimized conditions(a) Denitrification features
An example of results confirming denitrification3.
performance using electrolysis is shown in Fig. 3
Denitrification speed was confirmed to be almost constant (zero reaction), regardless of the ammonia
concentration in the solution.
In actual system operation where nitrogen (ammonia-nitrogen) is removed rapidly, real-time data on
treatment progress through ammonia concentration
is difficult to obtain. Hence, the behavior of the concentration turning to upward trend from downward
trend when the entire ammonia is treated was used
as an index to determine treatment progress, and electrolysis conditions were based on concentration (pH)behavior.
This enables electrolysis conditions (time and electrolytic current) to be set without being too high or
too low.
(b) Automatic control based on inlet concentration
Using control based on pH behavior, automatic control was implemented based on variations in nitrogen(ammonia-nitrogen) content in the treatment solution.

Electrolysis end
100

Next batch
electrolysis start

7.3

pH

7.1

Current (value)

80

NH4-N

6.7

50

6.5

40

6.3

30

6.1

20

5.9

10

5.7

pH

6.970

pH

NH4-N (m /L)Electrolysis
end

7.590

60

Electrolysis
end

B
Ats

5.50Time

B
Next batchcurrent(value)
Ats

Time

Time (min)Fig. 4 Setting pH variation with time and batch current

Fig. 3 Correlation between nitrogen (ammonia-nitrogen)concentration and pH in electrolytic treatment

Electrolytictank

Night soil
septic tanksludgePre-treatment

Biotreatment(nitrogen anddenitrificationtreatment)

Membranefor biotreatment

Mixing andcoagulation
treatment

Membraneforcoagulation

Treatment for
absorption ofactivated carbon

Discharge

Sludge treatment
equipment

Fig. 5 Application flow

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 4 (Nov. 2005)3Beforetreatment

Aftertreatment

Fig. 7 Color change in treated solution after electrolytic treatment

5. Conclusions

Fig. 6 Example of electrolytic denitrification delivery

MHI's new electrolytic denitrification has excellent
performance. Through hybrid operation withbiotreatment, it is widely applicable to wastewater containing organic substances. Its application to inorganic
wastewater such as plant wastewater, etc., despite differences in operation control, is expected to bewidespread.
Since denitrification performance largely depends on
symbiotic substances and chlorine ion concentration in
the treatment solution, the new electrolytic denitrification system is considered optimum treatment based on
the properties of the treatment solution. In addition to
package contracts including installation, lease contracts
including maintenance and supply of spare parts are alsoavailable at system introduction. MHI offers a wide variety of treatment systems as a solution to environmental
problems.
With legal regulations against nitrogen emission increasing for water quality conservation, we viewelectrolytic denitrification as a key technology for enhanced, stable denitrification. These systems also provide
sterilization and decoloration and are expected to beapplied to other substances as the optimum total solution to meet diversified needs.

Table 3 Example of operation data (Unit: mg/L)

Item

Unit inlet

Unit outlet

NH4-N

84.0

<0.5

NO2-N

0.9

<0.1

NO3-N

1.9

5.9

CODMn

12993(c) Operation record
Table 3 is an example of operation data. Salt water is added and the chlorine ion concentration in thetreatment solution is adjusted to 2 000 mg/L to ensure rapid denitrification.
Results show linear reduction of ammonia NH4 -N
concentration due to electrolysis treatment, with
gradual reduction seen also in the COD Mn concentration, while nitrous acid NO2-N and nitric acid NO3-N
concentrations remaining almost unchanged.
Color components are decomposed with brown
treatment solution becoming nearly colorless aftertreatment, suggesting a dramatically reduced load ofFig. 7
activated carbon in subsequent stages (Fig.
7).

Masamichi Asano

Hiroshi Mizutani

Kenji Nakamura

Takashi Ike

Yuudai Katou

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 4 (Nov. 2005)

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