With the population growing and more and more people wanting to live in rural communities, smaller towns and companies are installing decentralised sewage treatment plants (STPs) to solve their treatment issues. These systems are great in that they are a viable long-term alternative to centralised sewage treatment facilities but more often than not they are not maintained or operated correctly which results in licence breaches.
The article below lists five common problems with Sewage Treatment Plants (STPs) and how you can fix them.
1. Downstream equipment is failing due to a build of large solids, hairs and fibres
Looking at this you have to ask yourself, is your primary treatment adequate?
Screening is the first step in any wastewater treatment process. Screens remove the large non-biodegradable and floating solids that frequently enter wastewater works, such as rags, paper, plastic, tin, containers and wood.
Efficient removal of these materials will protect the downstream plant and equipment from any possible damage, unnecessary wear & tear, pipe blockages and the accumulation of unwanted material that will interfere with the wastewater treatment process.
Wastewater screening is generally classified into either coarse screening or fine screening. Screens may be manually or mechanically cleaned, with only the older and smaller treatment facilities using manually cleaned screens as their primary or only screening device. Screening products like REKO or Salsnes are essential to all WWTP’s and they will go a long way to improving the reliability of your downstream equipment.
2. Your treated effluent is not meeting Total Nitrogen (TN) targets
In order to determine where your problem is, the first step should be to analyse for Ammonia, Nitrite and Nitrate and thus this will give you an understanding of what area you need to focus your efforts on in order to make your plant compliant. More often than not you will find that your plant has either high Ammonia or Nitrate levels which contribute to the high TN levels.
Ammonia removal is a strictly aerobic biological process. If you have high ammonia look to ensure your plant has the following: –
- Generally, nitrification occurs only under aerobic conditions at dissolved oxygen levels of more than 1.0 mg/L
- Nitrification requires a long retention time
- A low food to microorganism ratio (F:M)
- A high mean cell residence time (measured as MCRT or Sludge Age)
- Adequate pH buffering (alkalinity)
The biological reduction of nitrate to nitrogen gas is performed by bacteria that live in a low-oxygen environment. To thrive, the bacteria need biochemical oxygen demand (BOD) – soluble BOD. Particulate BOD needs to be broken down into solution before it is of value.
If you have high treated effluent nitrate levels it is usually because of one of the following reasons. Ensuring these parameters are correct will go a long way to reducing your nitrate levels.
- Adequate carbon source. Denitrifying bacteria requires a considerable amount of soluble BOD (some five times as much as the amount of nitrate being denitrified) and many facilities find it difficult to provide an ongoing supply of readily digestible BOD
- Wastewater cannot be denitrified unless it is first nitrified. Ensure that the nitrification process is working otherwise there will be no nitrate to denitrify
- Ensure that the anoxic tank has a DO = 0.0mg/l. If this reading is higher than 0mg/l, the Mixed Liquor Return Rate (MLR) might need to be reduced but as a rule of thumb this flow rate is usually set at 300% of the daily incoming flow
3. Your treated effluent is not meeting Total Phosphorus (TP) targets
The primary objectives for the treatment process as it pertains to total phosphorus are:
- To achieve the license limits for TP
- To minimise total chemical usage
- To realise lower operating costs
In order to reduce TP there are a number of chemical dosing options that can be pursued and these should be considered once jar testing has been conducted, as this is a critical first step that will provide a benchmark for the execution of phosphorous measurement and treatment for the wastewater facility. It is essential at the outset to evaluate pre- and post-precipitation, and jar testing provides a quick analysis to help determine the most effective chemical needed for your process (e.g., alum or ferric chloride).
- Pre-precipitation. When identifying a chemical dosing point before the biological process, phosphorus is removed in the primary settling tanks. In conjunction, the online phosphate measurement should be taken between the primary settling and aeration tanks to be used as part of a system based on feedback control
- Post-precipitation. When chemical dosing is done after the biological process, phosphorus is removed in the final clarifiers or effluent filters. The online phosphate measurement should be taken between the aeration tanks and the final clarifier, or after the final clarifiers with feedback control
- Simultaneous (pre- and post-precipitation). This option utilises chemical dosing before and after the biological process, which facilitates low effluent phosphorus limits
4. Your treated effluent is not meeting biochemical oxygen demand (BOD) targets
Biochemical oxygen demand (BOD) is a measure of how much oxygen is required to biologically decompose organic matter in the water. Generally, when BOD levels are high, there is a decline in dissolved oxygen (DO) levels. This is because the demand for oxygen by the bacteria is high and they are taking that oxygen from the oxygen dissolved in the water. If there is no organic waste present in the water, there won’t be as many bacteria present to decompose it and thus the BOD will tend to be lower and the DO level will tend to be higher.
High effluent BOD levels in the treated effluent can have a number of causes some of which are shown below. Ensuring these are parameters are correct will go a long way to reducing your BOD levels.
- Incomplete wastewater treatment due to organic overloading
- Low oxygen concentration
- Low hydraulic detention time
- Physical short circuiting
- High algae or sulphur bacteria growth
- Sludge accumulation and loss of old sludge to the effluent
5. You have a lot of complaints as a direct consequence of odour issues associated with the WWTP
Odour is primarily due to the presences of hydrogen sulphide, but can also include mercaptans and organic acids that lend themselves to growing sulphur reducing bacteria. The bacteria generates hydrogen sulphide (H2S), a colourless gas with a foul, pungent odour similar to rotten eggs. Hydrogen sulphide gas is extremely corrosive and therefore needs to be removed and treated in order to prevent downstream equipment failures.
The first step in solving any odour problem is identifying the source. Odours could be coming from raw wastewater exposed to air at the influent pump station and primary clarifier stage. Unfortunately, the odours could also be coming from nearly any other step along the treatment train. Since the culprit could be a number of different things, or some combination thereof, it’s best to pinpoint the source of odours with the help of a professional consultant like OdaTech who can select an odour control technology ideally suited to your plant requirements.
As you can see it takes a lot to run a sewage treatment plant effectively to ensure it meets local discharge regulations. The problems and potential solutions identified above will hopefully give you a better understanding of how to correct some issues but ultimately selecting the right STP at the start of the process goes a long way to ensuring the plant is easy to maintain and it is fit for purpose. For more information on how to select the best technology please refer to our blog article here.
MAK Water proudly provides the commercial and industrial sectors with water treatment solutions compliant with local regulations. Our reliable and efficient industrial reverse osmosis systems, water and sewage/wastewater treatment systems have been assisting operations throughout Australia and internationally for over 20 years, as well as in South East Asia and South America.
At MAK Water, we are dedicated to helping industrial operations with their water, wastewater and sewage requirements, via the provision of high performing water treatment plants and systems. We have developed a reputation for delivering superior service and solutions, and for enabling our clients to reduce operating costs and extend equipment operating life.
What water treatment means with MAK Water:
- System design
- Project management
- Custom manufacturing
- System Servicing
- Remote monitoring
- Technical support
- Plant operation
- Ongoing equipment maintenance and support
Brendan Doherty, MAK Water Operations Manager WA