There are many different sewage treatment technologies in the UK. This guide will help you understand these technologies and focuses on the different styles of treatment plant to help educate you.
It is helpful to first break them down in to 2 categories as follows:
Treatment plants are designed to grow bacteria, which feed upon contaminants in wastewater producing cleaner water which should be safe to release into the environment. This bacteria can be freely floating in the water (active sludge), or fixed and growing on a surface, like filter media (Fixed media). This knowledge is important to help you understand the different types of treatment plant available.
In all active sludge systems, the bacteria live within the water and form “active sludge”. While activated sludge looks like a brown muddy substance, it is really the good bacteria that feed upon the contaminants in your wastewater (kitchen sinks, baths, showers and toilets).
SBR Systems also fall into the category of active sludge, but their treatment process is different so I will talk about them separately.
SBR treatment plants use active sludge, however it uses a timer to aerate the water and then let the bacteria settle in the chamber before decanting (often with a pump or airlift). This removes the need for a final settlement tank and adds the benefit of lowering the amount of time the air blower needs to operate). Furthermore, it offers the ability to adjust oxygen levels in the plant by changing the timer. This is a very elegant solution as you only use as much power as is needed to treat the wastewater
Fixed biology refers to systems where bacteria live on a surface (as opposed to living freely in the water). The surface may vary from system to system.
Within the Fixed biology system category there are a 4 sub categories:
RBC Stands for Rotating biological contractor. Whilst this is still classed as fixed biology, because the bacteria live on the discs, the discs slowly rotate to allow the bacteria some contact with the water and some contact with air, this provides the bacteria with its food, and oxygen).
Trickle filter systems were commonplace in the sewage treatment industry many years ago. Less so now but there are a few designs out there. Trickling filter systems have one thing in common, the waste trickles down through the filter media where the bacteria live. All energy free systems are trickle filter systems and some powered systems are still available (where water is brought up from the bottom of the tank and sprayed over filter media. This is often done using an “air lift”. An air lift is a simple way of moving water, an air compressor blows air into a pipe which is housed vertically in the water, as bubbles rise in the pipe, a small opening allows water to come into the pipe and rise with the bubbles. This is how some trickle filter systems lift and spray water over the filter material.
You'll notice the aforementioned systems were either powered, or energy free (non electric). Water either falls down thrown the media in the tank, or is regularly lifted and sprayed over the media. you may feel that energy free is better, however, the powered versions allow the cleansed water to be released from the tank near the top of the tank(near the ground). energy free systems are great, however because water has to flow down through the media, the outlet is therefore at the bottom of the tank. This often doesn't work for many sites that lack the natural fall on the land. This would lead you to need a pump to lift the water back up. It is a consideration when choosing this style of system.
A moving bed biological reactor is a treatment plant that has filter “media” floating around in the treatment tank. Bacteria live on this filter media. The concept is that when the media hit each other, any weak bacteria will fall off, leaving the strong to survive.
Systems that use MBBR:
SAF systems are designed, like the MBBR, in 3 stages, primary settlement, treatment zone and final settlement. The difference between SAF and MBBR is the filter media. In a SAF system the filter media is tightly packed and does not tend to freely move around like MBBR systems.
Systems that use SAF principles:
On a domestic level it really doesn’t make much difference which system you go for. But I tend to find that bacteria live longer on fixed media system in the event there is no wastewater vs active sludge systems, but active sludge systems seem to offer more aggressive treatment for a smaller tank volume. This being said, there are some very small activated sludge systems what may not provide as good treatment as systems designed with larger tanks. Too large a treatment volume however, can pose problems in any given system, so getting the balance right is important.
It is impossible to review all of them and compare all of them so this guide is just to point out the differences. Be aware though, a small tank may mean the manufacturer has reduced the sludge storage capacity, this is something I see with budget systems (along side cheap air blowers and a questionable tank volumes).
Active sludge system often offer slightly easier diffuser maintenance because there is no filter media in the way. Occasionally aerated filter media systems can suffer from dead zones where air is channeling up through the filter media via one route. I tend to try and agitate the media during servicing to prevent this and improve treatment. Active sludge systems don’t tend to suffer from this.
Furthermore, It is easier to restart an activated sludge system in the event a consumer hasn’t emptied their treatment plant. If sludge build up enters a the filter media of a fixed biology treatment plant, it can be very hard to clean off the filter media to gain effective treatment again.
Keeping either style of treatment plant working and treating well requires maintenance, and preferably from an experienced engineer who isn’t just coming to check if your air blower is working.