REASONS FOR

SUB OPTIMAL PERFORMANCE OF

UASB PLANTS-1

• Anaerobic treatment processes including UASB technology are very sensitive to environmental changes.

• Intermittent feeding can greatly affect the performance of a UASB reactor, as the anaerobic bacteria are sensitive to shock loading. This happens frequently at most of the places due to power cuts.

• Auxiliary power back-up facility is required at all the intermediate (IPS) & main pumping stations (MPS) of all the STPs.

--- REASONS-2

• It is observed that higher percentage of inert suspended solids that enter UASB has a direct impact on steady state VSS to TSS ratio in the reactor and ash content to the tune of 60% are common in UASB reactor.

• Therefore, proper operation of grit removal facility is very important to improve performance of UASB reactors.

-- Reasons-3

• Performance of polishing ponds, which is the terminal unit of the scheme, is also very crucial in deciding overall performance of the plant.

• Many polishing ponds have been found releasing TSS higher than an expected value of <30 mg/L. The combined effect of above factors often results in a final effluent having BOD >20 or 30 mg/L.

• Inadequate operation of the plants based UASB with Polishing Pond technology is gradually leading to development of a bad impression about the technology itself, which otherwise is the most suitable option for sewage treatment in our country.

Reasons-4

• The UASB followed by polishing pond system

does not give desired TSS outlet TSS.

• This indicates that in spite of a larger settling

area available in ponds as compared to

secondary clarifiers, fewer percentage of

ponds are able to provide effluent having TSS

less than 30 mg/L.

• Most obvious reasons behind this discrepancy

appear to be excessive algal growth due to

stagnation and high weir loading.

Reasons-5

• Operation and maintenance of existing plants

and sewage pumping stations is also a very

neglected field, as nearly 39% plants are not

conforming to the general standards

prescribed under the Environmental

(Protection) Rules for discharge into streams.

• STPs are usually run by operators who know

only operation of pumps and motors. The

operational parameters are not regularly

analyzed hence the day-to-day variation in

performance is not evaluated.

Key to Successful Operation Most important factors for successful operation of such plants include

• Continuous uniform feeding to the plant,

• Proper removal of grit,

• Maintenance of design VSS concentration and VSS/TSS ratio in UASB reactor,

• Cleaning accumulated sludge from polishing pond after a year time,

• Avoiding stagnation of water in ponds to prevent excessive algal growth and

• Providing proper weir length and baffle in the outlet structure of polishing pond,

TROUBLE SHOOTING MEASURES FOR ANAEROBIC DIGESTERS

• The operator needs to know:• What each part of the system is supposed to do?

• How each process or piece of equipment operates normally?

• How to recognize abnormal conditions?

• Alternatives available when trouble develops.

• Briefly, to recognize the trouble one must know process / equipment work under normal conditions.

• The purpose is to present a ready and quick operational reference to process problems and their solutions.

• The illustration below works as a guide in troubleshooting. The process is upset when the indicators show changes from normal. The first step is to find the correct problem. Is it loading, temperature, mixing, gas system or toxicity? To find the problem, as shown in Fig. proceed in a logical step by step fashion through thepossible cause blocks and eliminate all of theNO answers. This procedure allows the operator to check the whole system to correct problems and not symptoms.

• When the problem is found, the second step is to adopt following Troubleshooting Measures (TM) for the best corrective response.

• PROCESS UPSET INDICATOR* Decrease in gas production * Rise in Volatile Acid (Rise in VA/ALK ratio)* Decrease in pH* Decreasing COD reduction* Sudden increase in suspended solids in treated effluent

• PROCESS CAUSES: - Check following until cause in found.

• NOTE: Several items may occur simultaneously.

• If YES see TM1

• Loading * Loading rate has changed* Influent COD has changed* Flow of wastewater is increased* PH of wastewater has changedIf NO see next

• If YES see TM2

• Temperature * Reactor temperature changes more than 2°C/day* Unable to maintain reactor Temperature. If No see TM2

• If YES see TM3

• Mixing* Re-circulations pumps not working to design duty conditions.If NO see next

• If YES see TM4Gas System* Leaking gas * Excess pressure* Low pressure * Poor gas quality

• If NO see next

• If YES see TM5Toxicity

• * Slug does of toxic material

• * Feed has reached toxic limit

TM-1

• Cause: Every reactor is designed for a specific loading rate in terms of Kg COD (kg BOD) applied per day unit volume of reactor.

• Whenever, the influent loading rate increase beyond this, some after effects may be observed in the performance of the reactor.

• The loading might have increased either due to increase in flow (m3/day ) at the same designed influent COD concentration or flow remaining same, influent COD concentration is increased.

• Remedy: 1. Neutralize excess volatile acids by sodium bicarbonate

• .

2. If it is possible to dilute the reactor contents maintaining reactor temperature in desired range, add water along with effluent.

3. Decrease the feeding rate and starve the reactor for some time.

Cause: -pH of the wastewater is too low. The reason may be contamination by inorganic acids by accidental spillages in the industry premises or by any reason.

Remedy: -Neutralize the wastewater before feeding to reactor and adjust its pH around 7.0.

TM-2

In winter, the temperature of the reactor content may go down due to heat losses taking place especially during nights. It may be due to lower temperature of wastewater fed to the reactor, in summer, temperature of the reactor content may go above 40°C due to higher ambient temperature.

The anaerobic treatment process is highly sensitive to temperature. The bacteria responsible for this treatment do not work so efficiently both below or above the optimum temperature range and accordingly, reactor performance is affected.

• Remedy: -• 1. Temperature of the wastewater is the only controlling means to

maintain he reactor temperature within optimum range.• 2. If reactor temperature is going down, increase the temperature of

wastewater being fed to the reactor so that the reactor temperature can be raised and maintained within the optimum range.

TM-3

• Mixing is not adequate.• Mixing is one of the very important requirements for

achieving projected performance of the reactor as it brings bacteria and substrate (food) in animate contact with each other.

• Here mixing is mainly achieved by the rising liquid in the tank and release of gas bubbles. If the influent flow is not uniformly distributed mixing is advesely affected.

• Remedy:-• Routinely check if the liquid in tank is rising from all

sections of the reactor. Non uniform rising indicates blockage od\f some popenings in the influent pipe.

• The influent pipe cleaning can be done by starting the pump and opening the opposite side valve.

TM-4

• Cause: Gas Leakage in gasholder• Gas may be leaking through pressure relief valve on roof. This may

be due to valve not seating properly or is stuck open.• Remedy:• Close the isolating valve and remove PRV cover. Move weight

holder until it seats properly install a new ring if needed.• Cause:• Reactor gas pressure above normal.• Reactor PRV may have been stuck shut.• Water may have been accumulated in pipelines.• Remedy:• Close the isolating valve. Remove PRV cover and manually open

valve, clean valve seat. Drain moisture traps. Check for low-level spots.

TM 5 & 6• Cause:• Reactor gas pressure below normal.

• Too fast withdrawal causing a vacuum inside reactor.

• Remedy: Check vacuum relief valve for its operation.

• This will not affect reactor pressure if separate gas holder is provided• Cause: Poor gas quality (higher CO2 content).

• This may be due to process problems encountered by higher loadings.

• Remedy:

• Refer measure 2 for corrective action.

• TROUBLE SHOOTING MEASURES-6)

• Cause: -Wastewater unless contaminated with foreign waste is not excepted to give toxicity problems but if surface washing water containing insecticides, phenols or detergents are mixed with wastewater, toxicity may be developed in the reactor.

• Remedy: -

• 1. Dilution of the reactor content by water.

• 2. Prevent and stop contamination of wastewater by foreign waste

ACKNOWLEDGEMENT

• This presentation is based on the pioneering

work done in the field of Anaerobic digestion by

• Imhoff

• Perry McCarty

• Lettinga, &

• Arceivala

• The author gratefully acknowledges their

scholarly work in preparation of this presentation