Ecological sewage treatment

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Permalink Reply by Geert Dierick on October 3, 2008 at 0:02

Hi Deirdre

I'm working as a wastewater engineer here. Not promoting to be an expert, I'd rather see myself as someone with common sense in sewage.
If you're talking drying sewage, you will probably mean drying the sewage sludge, most likely coming from a mechanical sewage treatment plant (like an activated sludge, the classic one).

The sludge is indeed one of the biggest problems of the current sewage treatment schemes (together with the power requirements). I've only been for 1.5 years in the country but I have the impression that a lot of councils do not really have a very well managed sludge plan.
The practice over here (Far North) is dumping the sewage sludge in ponds (either the treatment ponds or separate ponds). If those get full, they will normally put the brown-black sticky sludge into geobags (large permeable sacks) that are left in the open to drain and dry by the sun. Once sufficiently dry, they are buried, apparently bag and all. Even this low tech approach can be very expensive. That's the major reason why a lot of councils delay the desludging of the ponds, which leads to reduced treatment performance.

The route that your region is taking seems quite a drastic one. Most likely they will first dewater the sludge (with a sort of mechanical presses) and further dry the dewatered cake with heat (currently using diesel). They probably already have that equipment in place (dewatering and the dryer). That's why the shift to wood instead of diesel is a fairly small one for them. They probably only have to do minor changes to the dryer when using wood.

Asking them to jump onto a completely different process will involve major capital input. Our council is always talking about depreciation of assets and other mumbojumbo. It basically comes down to this: they paid for a kit and want to use it until it's worn, you lose part of your investment when you throw it away halfway it's lifetime.

There are lots of other ways to deal with sludge: aerobic or anaerobic digestion, chemically stabilising it (lime), composting. Put for all these things you'll need new capital investment.


On the more general picture. In view of peak oil and climate change, our current centralized and highly mechanical wastewater treatment is completely unsustainable. This practice is far from resilient. Just imagine how we will keep those things running when the consequences of peak oil come about.
Will we be able to rely on the electrical power, the complex spare parts, the trained personnel, the chemicals, the money?

I've been teached this engineered wastewater treatment, and I believed in it for a few years. But I've seen the business now, a lot of people are passionate about it and they're good in what they're doing. But they don't seem to take a step back, look over it and gaze in the future. Most of my colleague wastewater engineers advocate for more technical stuff, the newest processes, the most high tech features, the most complex things they can come up with.

What we need for the years ahead is a robust and simple approach to dealing with our wastewater. My view is that the only way forward is to make people responsible for their own waste. We need to decentralize wastewater treatment and urgently go to localized and preferably individual treatment. I strongly believe that one of the best ways forward is the following: composting toilet for human waste (nearly all the potential pathogens are in the faeces, indigenous people knew this for centuries, never let this touch the water), urine separation at the stool (90% of the nitrogen is in the urine, superb fertilizer and free) and soil application or small wetland for the grey water (after simple grease trap and filter, grey water is bath, shower, washing machine and sink, fairly diluted, nearly no pathogens, a bit of phosphorus).

We'll need a change in mindset for this. Dump the word wastewater and call it resource, plant food.
A member of our TT here is organizing a trip around some local composting toilets, for council and local decision makers. What we hope to achieve is an action group, funded by coucil, to explore the benefits of the local approach.

Enough talk for me,
Regards,
Geert

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Permalink Reply by Deb Magon on November 3, 2008 at 8:52

Hi Geert,
We have a bio cycle system with a large drainage bed. We were told by the company we shouldnt grow anything edible on the drainage bed as the resultant grey water although fairly pure wasnt considered 'clean' enough for edible foods. Can you please explain to me how human waste can be used for fertilizer and what treatments etc it must go through. I have always been told it is too full of germs and must never be used for health reasons. Cheers Debbie

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Permalink Reply by Geert Dierick on November 3, 2008 at 23:58

Hi Deb

with the risk of becoming the "wastewater advisor" here, a few thoughts on your questions.

I'm not familiar with the bio cycle system, but I guess it's an aerated system + drainage bed. Your grey water has a low chance of containing human waterborne pathogens. The grey water comes typically from personal hygiene (bath, shower, sink), kitchen (sink, dish washer, ...) and washing machine. Still, there is a chance that any of these sources came into contact with human pathogens (e.g. small amounts of faeces or blood).
That company cannot take that risk and must advocate to people not to use the water for growing food. And according to me, that is a wise stance.
This grey water contains about 30% of your total phosphorus load and a small fraction of the total nitrogen load. You can easily use the drainage bed to grow compostable plants to recycle these nutrients. The compost will also increase the organic content in your edible garden and reduce the need for irrigation.

About human waste. Nearly all pathogens in typical combined sewage (faeces, urine and grey water) is contained in the faeces. About 100 years ago, a lot of cities collected the latrine waste during the night (night soil workers) and applied it straight on the surrounding farm land. In those days, epidemics were quite common also.
A lot of indigenous people, including the Moari, have learned the golden rule: never let human waste touch the water. Once the pathogens are in the water, they have their ideal vehicle to find their next host.

Instead, contain it, let them grow old and die, before you use it again. A composting toilet will do just that. It isolates the solids, which will undergo a cool, fairly slow composting process. In this process, bacteria and fungi will degrade and stabilize all the organic material present. The end product is compost or humus, which is basically the skins and bones of the original faeces, a very stabilized organic end product, which is very hard to digest for any micro-organism. This material will increase the organic content of your soil, it takes years to completely degrade it, it has a high moisture capacity and a high nutrient absorption capacity. It's a sponge for all plant roots to tap in.

As for the original pathogens in the faeces. These organisms cause disease because they grow and multiply inside our bodies. As far as I know, all the human pathogens require either an animal or human host to multiply. The composting toilet is not their favourite spot. There is a huge competition and predation from all the other organisms in the composting heap. The pathogens will decline exponentially, due to lack of growth factors (host), predation, hostile environment (can be oxygen, ammonia, temperature, antibiotics produced by fungi, ...) and simple ageing (they have to consume their reserves in order to live).
A conservative rule is to keep human compost 2 years isolated before use for food crops.
Personally, I would keep it at least a year isolated and use it only for non tuber food crops (so not for potatoes, carrots, ...). This way, the compost will get the final disinfection in the soil (huge microbial life there, including lots of fungi, some of whom excrete antibiotics) and the risk of contaminating the edible part of the crop is reduced to the minimum.
Another option is to mix the human compost into a good, hot garden compost heap. Provided it is a hot heap (temperatures > 50 degrees C.), it will thoroughly disinfect the human compost.

A few figures to finish. Typically, faeces contains 50% of the organic waste, 30% of the phosphorus and 5% of the nitrogen of a total household wastewater. 90% of all the nitrogen of that household is in the urine. The other 50% of the organic waste is in the grey water.
A person produces 150 g faeces/d (wet), with a dry solid content of 45 g/d. A family of four will produce 600 g/d, wet. A composting toilet will typically reduce to between 10 and 30% of the original volume, take 30%. Assume faeces has a density of 1 kg/L, sometimes it floats, sometimes it sinks. After 1 year, this family will have produced 65 kg human compost. Put another 50% on top due to use of bulking materials (toilet paper, wood chips, other stuff). Brings you to around 100 kg human compost per year, for 4 people. Ten buckets.

If you're interested, there are some interesting books available. "The composting toilet system book" is one, see Amazon.

Cheers,
Geert

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