  We present deep spectrophotometry for a sample of 8 PNe and 12 HII regions in
the irregular galaxy NGC 3109, to analyze the chemical composition of both
types of nebulae. We present line intensities and the physical conditions and
the abundances of He, O, Ne, N, S and Ar are derived, using the classical
T_e-based method. We confirm our previous identification of PNe and HII regions
based on photometry, except for one object that we argue is a compact HII
region rather than a PN. We find that the chemical composition of the ISM in
NGC 3109, as sampled by its HII regions, is remarkably uniform. The oxygen
abundance is log O/H + 12 = 7.77 \pm 0.07 in this galaxy, as compared to 8.05
\pm 0.09 for the SMC. PNe show significantly higher oxygen abundances in NGC
3109: log O/H + 12 = 8.16 \pm 0.19. We argue that, similarly to what has been
suggested for some of the PNe in the Magellanic Clouds and other metal-poor
galaxies, oxygen in the PNe in NGC 3109 is affected by dredge up in their
progenitors. This could be also the case for neon. From our analysis, we
conclude that these two elements are not always a safe indicator of the
chemical composition of the ISM at low metallicities. An alternative to the O
and Ne enrichment in PNe is that the low metallicity in HII regions has been
caused by dilution of the ISM due interaction with a neighbor galaxy about a
Gyr ago.
  The excitation patterns of the PNe in NGC 3109 are very different from the
excitation patterns of PNe in other galaxies. This would imply that the
evolution of PNe depends upon the properties of their progenitor stellar
populations, which vary from galaxy to galaxy. This should affect the PN
luminosity function and its use as a distance indicator.
